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'.Publishers  of  Books  for" 

Electrical  World  The  Engineering  and  Mining  Journal 

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Metallurgical  and  CKemical  Engineering 


ELECTRICAL  CATECHISM 


AN  INTRODUCTORY   TREATISE   ON 
ELECTRICITY  AND  ITS   USES 


BY 


GEO.  D.  SHEPARDSON,  M.E. 

ii 

Professor  of  Electrical  Engineering  in  the  University  of  Minnesota 


SECOND  EDITION,   CORRECTED  AND  REVISED 

SECOND    IMPRESSION 


McGRAW-HILL    BOOK    COMPANY 

239  WEST    39TH   STREET,   NEW  YORK 

6  BOUVERIE  STREET,  LONDON,  E.C. 

1908 


Engineering 
Library 


COPYRIGHT,  1901, 

BY 
AMERICAN  ELECTRICIAN  COMPANY 

AND  1908, 
BY   THE 

McGRAW  PUBLISHING  COMPANY 
NEW  YORK 


PREFACE   TO    SECOND   EDITION. 


In  this  edition,  the  opportunity  has  been  taken  to  revise  many 
sections  that  had  become  obsolete,  to  correct  a  few  errors  which  crept 
into  the  earlier  edition,  and  to  add  considerable  information.  Changes 
of  greater  or  less  magnitude  have  been  made  in  over  two  hundred 
sections.  The  index  has  been  amplified  correspondingly.  More 
than  sixty  new  cuts  have  been  added  or  substituted  for  those  ob- 
solescent. The  Appendices,  added  in  this  edition,  will  be  found  to 
contain  much  information  in  convenient  form. 

The  writer  realizes  that  some  chapters  might  have  been  further 
enlarged  and  new  ones  added,  without  exceeding  the  scope  of  an 
introductory  discussion.  It  is  believed,  however,  that  the  work  as 
now  presented,  illustrates  the  practice  and  beliefs  of  to-day. 

Minneapolis,  September,  1908. 


254557 


PREFACE    TO    FIRST    EDITION. 


The  work  here  presented  is  a  revision  and  enlargement  of  the 
Electrical  Catechism,  of  which  the  first  instalment  appeared  in  Elec- 
trical Industries  in  February,  1895,  and  which  has  continued  without 
interruption  through  succeeding  volumes  of  that  periodical  and  its 
successor,  the  American  Electrician.  It  is  designed  to  answer  the 
numerous  questions  that  continually  come  up  in  the  minds  of  those 
who  come  into  contact  with  any  application  of  electricity.  The  topics 
are  selected  from  personal  inquiries  and  letters  and  from  the  queries 
noted  in  electrical  papers,  these  being  supplemented  by  others  in- 
tended to  prepare  the  way  and  to  make  the  treatment  more  con- 
secutive and  comprehensive. 

The  writer  has  aimed  to  present  in  simple  non-technical  language 
the  information  sought  by  electrical  workmen,  superintendents,  en- 
gineers, dynamo  tenders,  wiremen,  motormen,  inspectors,  mechanics 
in  repair  shops  and  factories;  to  lay  before  the  general  reader  the 
important  facts  relating  to  electricity  and  its  various  applications 
without  a  confusion  of  technical  terms  and  formulae;  and  to  give  the 
student  a  general  view  of  the  subject,  which  may  serve  as  an  intro- 
duction to  the  more  special  and  formal  treatises.  Little  is  presented 
which  could  not  be  found  elsewhere  if  one  knew  where  to  look  and 
had  the  necessary  time  and  facilities.  It  is  hoped  that  many  who 
have  not  the  time  or  opportunity  to  study  the  more  formal  books  will 
here  find  some  of  the  desired  knowledge  presented  in  convenient 
form  and  attractive  style.  The  copious  index  at  the  end  of  the 
volume  renders  the  whole  easily  accessible.  Limitations  of  time  and 
space  have  crowded  out  many  topics  of  interest,  but  the  material  pre- 
sented may  be  considered  as  reliable.  While  it  is  believed  that  no 
inaccurate  statements  are  made,  the  publishers  and  the  writer  will 
appreciate  the  notice  of  any  errors  that  may  have  crept  in  and 
escaped  notice. 

The  writer  takes  this  opportunity  to  acknowledge  his  obligations 
to  E.  L.  Powers,  formerly  editor  of  Electrical  Industries,,  at  whose 
request  the  series  was  begun;  to  W.  D.  Weaver,  who  as  editor  of  the 
American  Electrician,  urged  its  continuance,  and  to  whom  is  due  the 
excellent  collection  of  portraits  of  the  men  for  whom  units  were 
named,  some  of  these  appearing  now  for  the  first  time  in  print;  to  J. 
Zeleny  and  F.  W.  Springer  for  numerous  suggestions,  and  to  L.  Mc- 
Laughlin  and  others  for  much  helpful  and  patient  work  in  the  pub- 
lishers' offices.  Thanks  are  due  also  to  various  manufacturers,  whose 
trade  literature  has  been  drawn  upon  freely  for  illustrations. 

iv 


CONTENTS. 


Chapter  Numbers  PagC 

I.  Static    Electricity 1-158 

II.  Units    200-251  46 

III.  Laws  of  Electric  Circuits 300-376  62 

IV.  Electricity  and  Heat. 400-515  85 

V.  Batteries  and  Electrochemical  Action 600-  654  138 

VI.  Magnetism    7°°'  793  168 

VII.  Electrical  Measuring  Instruments 800-856  204 

VIII.  Electrical  Measurements QOO-  959 

IX.  Elementary  Motors 1000-1017  249 

X.  Dynamos  (Direct  Current) 1100-1269  258 

XL  Motors  (Direct  Current) 1300-1388  307 

XII.  Alternating  Currents    :......... 1400-1520  337 


CHAPTER  I. 


STATIC  ELECTRICITY. 

1.  What  is  electricity? 

Electricity  is  either  a  substance  or  a  force.  Some  scientists  be- 
lieve it  to  be  the  same  as  the  ether,  an  elastic  and  extremely  thin  sub- 
stance that  is  supposed  to  exist  everywhere,  even  between  the  mole- 
cules of  solids  and  liquids,  and  through  the  space  between  the  stars. 
Others  think  it  a  force  or  form  of  energy ;  others  a  condition  of  the 
ether.  Many  believe  it  to  be  the  fundament  of  which  the  elements 
are  modifications.  (See  Nos.  12,  644.) 

2.  What  does  the  zvord  "electricity"  mean? 

It  comes  from  a  Greek  word,  elektron,  meaning  amber.  The 
Greeks  at  the  time  of  Thales,  600  B.  C.,  knew  that  when  amber  was 
rubbed,  it  would  attract  bits  of  paper  and  straw. 

3.  //  ive  do  not  know  what  electricity  is,  how  can  we  use  it? 
Although  the  exact  nature  of  electricity  is  not  fully  understood, 

much  is  known  about  it.  Many  laws  have  been  discovered  that  al- 
ways hold  true.  We  do  not  know  exactly  what  gravitation  is,  and 
yet  we  know  its  laws  and  how  to  make  use  of  them. 

4.  How  many  kinds  of  electricity  are  there? 

All  electricity  is  probably  one  and  the  same.  (See  also  No.  140.) 
It  is  sometimes  classified  according  to  its  motion,  as : 

(1)  Electricity  at  Rest,  or  Static  Electricity. 

(2)  Electricity  in  Motion,  or  Current  Electricity. 

(3)  Electricity  in  Rotation,  or  Magnetism. 

(4)  Electricity  in  Vibration,  or  Radiation. 

5.  What  is  static  electricity? 

Static  electricity  consists  of  stationary  charges,  which  show  them* 
selves  by  attracting  or  repelling  other  bodies. 

6.  What  is  electricity  in  motion? 

An  electric  current  manifests  itself  by  heating  the  wire  or  con- 
ductor, by  causing  a  magnetic  field  around  the  conductor  and  by 
causing  chemical  changes  in  a  liquid  through  which  it  may  pass. 
It  is  now  believed  that  the  electrical  energy  is  not  carried  through 
the  wire,  but  through  the  space  around  it 


2  ELECTRIC  A  L  .  CA  TECHISM. 

7.  What  is  electricity  in  vibration? 

When  the  current  oscillates  or  vibrates  back  and  forth  with  ex- 
treme rapidity,  it  takes  the  form  of  waves  which  are  similar  to 
waves  of  light. 

8.  Is  magnetism  a  kind  of  electricity? 

Magnetism  is  a  result  of  electric  currents  and  under  some  con- 
ditions will  produce  currents.  A  common  theory  is  that  magnetism 
is  caused  by  currents  of  electricity  circulating  in  the  molecules  or 
particles  of  a  magnetized  substance. 

9.  How  long  has  anything  been  known  about  magnetism? 

Hoang  Ti,  a  Chinaman,  is  said  to  have  used  a  magnet  for  a  com- 
pass in  2637  B.  C.  The  Greeks  used  a  magnet  or  lodestone  at  the 
siege  of  Troy  about  1000  B.  C. 

10.  How  long  ago  was  static  electricity  discovered? 

It  is  not  known.  Moses  was  acquainted  with  the  lightning,  and 
some  of  the  vessels  in  the  tabernacle  seem  to  have  been  made  with  a 
knowledge  of  electricity.  The  Temple  at  Jerusalem  was  well  pro- 
tected against  lightning  and  was  not  struck  in  1000  years.  A  Greek 
philosopher,  Thales,  about  600  B.  C.,  mentioned  that  amber  (elek- 
tron)  attracted  bits  of  straw  when  rubbed.  Aristotle,  in  341  B.  C., 
wrote  about  electric  fishes  that  paralyzed  other  animals. 

11.  When  was  current  electricity  discovered? 
By  an  Italian  named  Volta,  about  a  century  ago. 

12.  When  were  electric  waves  discovered? 

Light  has  been  known  since  the  beginning  of  the  world^  Joseph 
Henry,  an  American,  proved  in  1842  that  the  discharge  from  a  Ley- 
den  jar  was  oscillatory.  It  was  not  until  1867  that  an  English  mathe- 
matician, Maxwell,  proved  mathematically  that  light  and  electricity 
were  the  same.  In  1888  Heinrich  Hertz  proved  by  experiment  that 
this  was  true. 

13.  Why  did  the  older  text-books  give  so  much  attention  to  static 
electricity? 

Because  more  was  known  about  it  than  about  currents,  and  be- 
cause it  was  a  means  of  making  many  attractive  experiments. 

14.  Is  static  electricity  of  any  importance  in  practical  work? 
Electrostatic  charges  are  important  with  pulsating  or  alternating 

currents  in  telegraphy,  telephony,  power  transmission  lines,  spark 
coils,  lightning,  etc.     (See  Nos.  101,  107,  115  116,  143.) 


STATIC  ELECTRICITY,  3 

15.  Why  should  a  practical  -worker  learn  about  static  electricity? 
Static  electricity  is  dangerous  in  some  cases  and  troublesome  in 

many  others,  and  one  should  know  how  to  avoid  or  get  rid  of  it. 
In  other  cases  it  is  useful,  as  in  the  application  of  condensers  to  tele- 
phone ringing  and  talking  circuits  and  in  making  possible  simul- 
taneous telephony  and  telegraphy  on  the  same  line. 

16.  In  what  different  ways  is  static  electricity  produced? 

By  chemical  action,  by  magnetic  induction,  by  heat,  by  friction,  by 
influence  and  in  some  cases  by  pressure. 

17.  How  does  static  electricity  shozv  itself? 

By  the  attraction  or  repulsion  between  charged  bodies.  When 
static  electricity  is  discharged,  it  causes  more  or  less  of  a  current, 
which  shows  itself  by  the  passage  of  sparks  or  a  brush  discharge ; 
by  a  peculiar  prickling  sensation ;  by  a  peculiar  smell,  due  to  its 
chemical  effects ;  by  heating  the  air  or  other  substances  in  its  path ; 
and  sometimes  in  other  ways. 

1 8.  Is  any  special  or  costly  apparatus  needed  to  detect  static 
electricity? 

It  is,  if  one  wishes  to  make  very  accurate  and  delicate  measure- 
ments. But  much  can  be  learned  by  very  simple  and  cheap  means. 

19.  What  are  some  easy  ways  of  producing  and  shoiving  the  ef- 
fects of  static  electricity? 

Rub  a  dry  sheet  of  paper  with  a  piece  of  dry  cloth  or  with  a  coat 
sleeve,  and  the  paper  will  stick  to  a  table  or  to  the  wall  for  some 
time. 

Pass  a  rubber  or  celluloid  comb  through  the  hair  on  a  dry  or 
cold  day  and  the  hair  will  stick  to  the  comb.  If  it  is  quiet,  one  may 
hear  a  slight  snapping  noise.  The  ends  of  the  hair  will  also  stand 
out  as  if  they  repelled  one  another.  If  it  is  dark,  one  will  sometimes 
see  small  sparks  from  his  hair,  like  the  sparks  from  a  cat's  back. 

By  shuffling  across  the  carpet  on  a  cold,  dry  day,  and  then  touch- 
ing another  person  or  a  gas  or  water  pipe,  a  spark  will  pass  from 
him  and  he  will  get  a  shock  at  the  same  time. 

20.  How  can  one  make  a  cheap  indicator  for  studying  static 
electricity? 

By  hanging  a  small  bit  of  pith  or  feather  on  the  end  of  a  fine  silk 
thread,  or  on  a  fine  hair,  several  inches  long. 

21.  What  is  such  an  indicator  called? 

An  electroscope,  because  it  enables  us  to  see  whether  a  body  is  elec- 
trified or  not. 


4  ELECTRICAL   CATECHISM. 

22.  How  is  the  feather  electroscope  used? 

If  an  electrified  body  is  brought  near  it,  the  feather  will  be  at- 
tracted. If  the  feather  touches  it,  it  will  soon  be  repelled  and  fly 
away. 

23.  How  can  a  more  sensitive  electroscope  be  made? 

Take  a  dry  bottle  with  a  wide  mouth ;  push  through  the  cork  a 
piece  of  wire,  the  lower  end  of  which  is  bent  so  as  to  form  a  square 
hook;  upon  this  hook,  hang  a  strip  of  ordinary  gold  leaf  (which 
may  be  obtained  from  a  dentist  or  at  a  bookstore,)  in  such  a  way 
that  the  two  ends  are  of  the  same  length  and  hang  flat  and  close  to- 
gether; place  the  cork  in  the  bottle  and  adjust  the  wire  so  that  the 
gold  leaves  do  not  touch  either  the  sides  or  the  bottom  of  the  bottle. 
If  the  electroscope  is  charged  by  touching  the  wire  with  some  sub- 
stance that  has  been  previously  charged,  the  two  gold  leaves  stand 
apart  as  if  repelling  one  another. 

24.  What  can  be  learned  from  the  gold  leaf  electroscope? 

It  will  show  whether  a  body  is  electrified ;  it  will  indicate  to  some 
extent  how  much  charge  it  has;  it  will  show  whether  the  charge  is 
positive  or  negative.  It  will  show  whether  wires  are  highly  charged. 

25.  What  is  meant  by  a  positive  or  negative  charge ? 

With  the  feather  or  pith-ball  electroscope,  it  is  noticed  that  the 
feather  is  at  first  attracted  to  the  charged  body,  and  is  then  repelled 
from  it.  If  one  charges  the  gold-leaf  electroscope  with  a  rubber 
comb  that  has  been  rubbed  with  a  dry  cloth,  the  leaves  will  stand 
further  apart  if  the  rubber  is  brought  near  the  wire  again ;  but  if  the 
rubbed  part  of  the  cloth  is  brought  near,  the  gold  leaves  come  closer 
together.  When  dry  hair  is  dressed  with  a  rubber  comb,  the  hairs 
are  attracted  by  the  comb,  but  they  stand  apart  from  one  another. 
This  shows  that  when  two  substances  are  rubbed,  both  are  electrified ; 
but  the  charges  are  different.  It  also  shows  that  bodies  oppositely 
charged  attract  one  another,  but  that  bodies  similarly  charged  repel 
each  other.  This  is  analogous  to  the  law  that  like  poles  of  magnets 
attract  one  another,  while  unlike  poles  repel. 

26.  What    is    the    difference    between   positive    and    negative 
charges? 

It  is  not  definitely  known.  Many  scientists  have  believed  that 
there  were  two  kinds  of  electricity ;  others  believe  that  there  is  only 
one  kind.  If  a  body  contains  more  than  the  usual  amount,  it  is  said 
to  be  positively  charged ;  if  it  contains  less  than  the  usual  amount, 
it  is  said  to  be  negatively  charged.  A  body  is  discharged  when 
the  positive  and  negative  electricities  come  together  and  are  neutral- 


STATIC  ELECTRICITY.  5 

ized.  According  to  the  other  theory,  a  body  is  discharged,  or 
neutral,  when  it  contains  neither  more  nor  less  than  the  usual 
amount.  If  electricity  is  a  state  of  strain  in  the  ether,  a  body  is 
charged  or  not,  accordingly  as  the  ether  in  and  about  it  is  com- 
pressed, stretched,  or  is  neither. 

27.  Can  electricity  be  produced  by  rubbing  any  two  substances 
together? 

When  they  are  unlike  and  are  insulated. 

28.  Does  the  same  amount  of  rubbing  always  produce  the  same 
amount  of  electricity? 

No.  It  depends  upon  what  substances  are  rubbed.  Some  are 
much  more  strongly  electrified  than  others. 

29.  Are  some  substances  always  positively  electrified  by  rubbing, 
and  others  always  negatively  electrified? 

When  any  two  substances  are  rubbed  together,  one  of  them  is  al- 
ways positively  electrified,  and  the  other  negatively.  The  same  sub- 
stance may  be  either  positive  or  negative,  according  to  the  nature 
of  the  other  substance.  Different  materials  may  be  arranged  in 
such  an  order  that  any  one  will  be  positively  electrified  if  rubbed  by 
any  substance  before  it  on  the  list,  but  negatively  electrified  if  rubbed 
by  any  substance  coming  after  it. 

30.  Can  static  electricity  be  produced  equally  well  by  rubbing  any 
two  substances  together? 

No.  It  is  difficult  to  find  any  by  rubbing  metals  together.  The 
presence  of  moisture  lessens  the  effects.  Substances  known  as  insu- 
lators become  electrified  easily,  while  those  known  as  conductors 
do  not. 

31.  Why  do  not  electrical  conductors  easily  become  electrified 
by  friction? 

Because,  as  fast  as  the  electricity  is  produced,  it  is  carried  away 
and  becomes  neutralized. 

32.  What  is  the  difference  between  insulators  and  conductors? 
Conductors  are  those  substances  in  which  electricity  travels  easily, 

while  insulators  do  not  furnish  an  easy  path.  There  is  no  clearly 
marked  line,  since  no  substance  is  a  perfect  conductor,  offering  ab- 
solutely no  resistance  to  the  motion  of  electricity;  nor  is  any  sub- 
stance a  perfect  insulator,  absolutely  preventing  the  motion  of  elec- 
tricity. Different  substances  may  be  arranged  in  more  or  less 
definite  order,  with  the  good  conductors  at  one  end  and  with  good 
insulators  at  the  other  end. 


6  ELECTRICAL  CATECHISM. 

33.  Give  a  list  of  some  common  substances  arranged  in  the  order 
of  their  conductivity. 

i,  silver;  2,  copper;  3,  other  metals;  4,  carbon,  charcoal  and 
graphite ;  5,  acids  and  salty  solutions ;  6,  animals  and  plants ;  7, 
pure  water;  8,  various  oils;  9,  cotton;  10,  dry  wood;  n,  marble; 
12,  porcelain;  13,  wool;  14,  silks;  15,  rubber;  16,  shellac;  17, 
paraffin ;  18,  glass ;  19,  dry  air. 

34.  Do  these  substances  always  keep  in  the  same  relative  order? 
No.     Their  conductivity  is  affected  by  various  conditions,  such  as 

their  purity,  the  presence  of  moisture  and  other  causes. 

35.  If  a  substance  is  a  conductor  for  static  electricity,  will  it  also 
conduct  current  electricity  f 

Not  always.  Any  substance  that  will  conduct  a  current  will  also 
carry  static  electricity.  Since  the  latter  is  of  high  pressure  and 
comparatively  small  quantity,  it  will  travel  slowly  over  surfaces 
that  would  not  carry  any  perceptible  current  of  electricity.  Static 
charges  will  gradually  creep  along  the  surfaces  of  dry  wood  or 
glass,  substances  which  are  excellent  insulators  for  currents.  In 
fact,  the  doctors  sometimes  use  wooden  electrodes  with  static  ma- 
chines in  order  to  give  gentle  diffused  discharges  to  patients. 

36.  Is  static  electricity  carried  through  the  body  of  a  conductor 
or  upon  its  surface  only? 

It  is  generally  understood  that  a  static  charge  is  only  on  the  sur- 
face. A  current  generally  spreads  through  the  conductor,  so  that 
the  conductivity  for  currents  varies  with  the  area  of  the  cross-section 
rather  than  with  the  area  of  the  surface. 

37.  What  is  conduction  f 

An  example  of  conduction  is  found  by  holding  a  wire  or  a  piece 
of  metal  pipe  near  a  belt  or  other  source  of  static  electricity.  If  it  is 
developing  much  electricity,  one  will  get  a  shock  through  the  pipe 
as  strong  as  from  the  belt  itself.  Sometimes  a  shock  will  be  re- 
ceived from  a  machine,  a  line  shaft  or  from  a  belt-shifter.  The 
electricity  comes  from  the  belt,  and  is  conducted  by  the  metal. 
Static  electricity  may  be  carried  considerable  distances  if  the  con- 
ductor is  well  insulated.  The  static  electricity  from  dynamo  belts 
sometimes  gets  over  the  insulation  on  the  machines  into  the  circuits 
and  is  carried  along  the  wires.  It  is  very  difficult  to  get  perfect 
insulation  so  as  to  prevent  the  escape  of  a  static  charge,  and  instru- 
ments for  measuring  static  electricity  must  be  designed  and  used 
with  great  care,  unless  the  escape  of  some  of  the  charge  is  not  im- 


STATIC  ELECTRICITY.  7 

portant.  An  example  of  conduction  of  static  electricity  is  sug- 
gested in  the  figure,  in  which  two  tin  pails  are  connected  by  a  wire 
and  are  suspended  by  means  of  dry  cords  or  threads.  If  one  pail  is 
near  a  belt,  a  shock  may  be  obtained  by  touching  the  other  pail.  If 


FIG.    37.— CONDUCTION    OF    ELECTRICITY. 

the  belt  is  not  generating  enough  electricity  to  give  one  a  shock,  it 
may  be  detected  by  the  repulsion  of  a  small  pith  ball  or  a  bit  of 
feather  held  near  the  pail  by  a  fine  thread. 

38.  How  is  static  electricity  generally  obtained  for  experimental 
purposes? 

It  is  generally  obtained  from  frictional  or  from  "influence"  ma- 
chines. The  former  are  common  in  the  older  laboratories,  but  are 
now  being  replaced  by  more  modern  machines,  such  as  the  Toepler- 
Holtz  or  the  Wimshurst  machines. 

39.  What  is  a  frictional  electric  machine? 

A  common  form  consists  of  a  circular  glass  plate  which  may  be 
rotated  and  upon  which  two  leather  or  felt  rubbers  press.  The  rub- 


ric. 39.— FRICTIONAL  ELECTRICAL  MACHINE. 

bers  are  coated  with  an  amalgam  of  tin  and  mercury.  When  the 
glass  plate  revolves,  the  friction  causes  the  rubbers  to  become  nega- 
tively electrified,  while  the  glass  becomes  charged  with  positive  elec- 
tricity, which  is  collected  by  a  number  of  sharp  points  on  a  sort  of 


8  ELECTRICAL   CATECHISM. 

metallic  comb  placed  diametrically  opposite  from  the  rubbers.  The 
comb  and  the  rubbers  are  connected,  respectively,  with  two  large 
knobs  called  "prime  conductors,"  from  which  sparks  or  a  brush 
discharge  may  be  taken. 

40.     How  do  Toepler-Holtz  or  Wimshiirst  machines  operate? 

Their  theory  is  too  complicated  for  full  discussion  here.  They 
depend  not  upon  friction,  but  upon  electrostatic  induction  or  "influ- 
ence." They  are  based  upon  the  fact  that  oppositely  charged  bodies 
attract  each  other,  hence  it  requires  work  to  move  them  apart. 
When  the  two  bodies  have  a  certain  charge  and  are  then  separated, 
the  work  of  moving  them  apart  increases  the  difference  between 


FIG.  40.— WIMSHURST  MACHINE. 

their  potentials.  An  initial  charge  is  given  to  the  machines  by 
some  convenient  method,  such  as  a  small  friction  machine  or  an 
electrophorus,  and  this  is  rapidly  increased  by  induction.  Such  in- 
duction is  probably  a  principal  source  of  the  electricity  of  the  clouds. 
Fuller  explanation  is  given  in  Mason's  "  Static  Electricity."  Direc- 
tions for  making  simple  machines  are  given  in  American  Electrician 
of  November,  1896,  and  August,  1897. 

41.  Do  dynamos  for  operating  electric  lights  and  motors  gener- 
ate electricity  by  friction? 

No.  These  are  entirely  different  from  frictional  and  other  static 
machines,  and  will  be  discussed  later  in  Nos.  noo  to  1270. 

42.  What  sort  of  a  frictional  machine  is  used  for  lighting  gas? 

Several  forms  have  been  on  the  market.  The  style  illustrated  con- 
tains a  hard  rubber  disc  that  is  rotated  by  means  of  a  crank.  One 
or  more  pairs  of  amalgamated  pads  are  pressed  against  the  disc. 
The  charges  on  the  pads  and  disc  are  conducted  into  a  condenser 


STATIC  ELECTRICITY.  9 

(see  Nos.  in  and  112)  that  forms  part  of  the  frame.  The  machine 
is  charged  by  turning  the  crank  several  revolutions  in  the  direction 
of  a  clock,  or  "clockwise,"  and  then  turning  it  back  a  short  distance. 
The  disc  and  the  frame  in  which  it  is  supported  have  a  certain 
amount  of  play  inside  the  case.  When  the  crank  is  turned  back- 
ward, the  two  terminals  of  the  condenser  come  into  contact  with  two 


FIG.  42.— FRICTIONAL  GAS-LIGHTING  MACHINE. 

binding  posts  on  the  case,  and  the  charge  in  the  condenser  causes 
a  momentary  current  to  flow  through  the  circuit  connected  between 
the  terminals  L  and  G.  Induction  coils  are  now  used.  (See  No.  49.) 

43.     Plow  is  the  discharge  used  to  light  gas? 
It  is  used  for  lighting  a  number  of  jets  supplied  by  the  same  gas 
pipe  which  may  be  in  places  difficult  of  access  for  lighting  with 


FIG.  43.-JUMP-SPARK  GAS  TIP. 

matches  or  tapers.  Each  gas  jet  has  a  special  tip  of  lava  or  other 
insulating  material  and  carries  two  wires,  which  are  separated  a 
short  distance,  and  are  so  placed  that  a  spark  passing  between  them 
will  ignite  the  gas  as  it  issues  from  the  tip.  The  wires  on  the  dif- 
ferent tips  are  connected  "in  series,"  so  that  the  discharge  from  the 
machine  will  cause  a  spark  or  number  of  sparks  to  jump  across  from 
wire  to  wire  on  each  tip.  The  wires  are  carefully  insulated  and  are 


10  ELECTRICAL  CATECHISM. 

so  placed  that  sparks  will  not  occur  at  any  place  except  at  the  gas 
tips.  In  operating  the  device,  the  gas  is  turned  on  and  the  crank 
is  turned  rapidly  and  then  turned  back.  By  this  time  the  gas  has 
filled  the  pipe  and  is  issuing  from  the  tips.  The  backward  move- 
ment of  the  crank  connects  the  machine  with  the  circuit,  and  the 
sparks  light  the  gas  simultaneously  at  all  the  jets. 

44.  Can  one  machine  be  used  to  light  more  than  one  set  of  gas 
jets? 

Yes.  For  this  purpose  a  "multiple  point  switch"  is  used,  by  which 
one  terminal  of  the  machine  may  be  connected  to  any  one  of  a  num- 
ber of  circuits.  It  is  customary  in  buildings  without  electric  lights 
to  "ground"  one  end  of  each  circuit  upon  the  gas  pipe,  which  thus 
becomes  a  common  return  for  all  the  circuits,  in  which  case  one 
terminal  of  the  machine  is  also  grounded  on  the  pipe.  Fig.  42 
shows  a  machine  connected  to  a  four  point  switch,  so  that  it  can 
light  the  gas  on  any  one  of  four  circuits. 

45.  Is  it  safe  to  use  the  gas  pipe  for  the  common  return? 

It  is  safer  to  use  a  separate  wire  for  the  return,  especially  where 
the  gas  lighting  device  is  used  in  a  building  lighted  by  both  gas  and 
electricity.  The  high  pressure  used  in  connection  with  the  spark 
circuit  is  liable  to  cause  a  spark  to  jump  to  the  other  circuit,  and 
so  occasion  risk  of  danger  from  fire.  The  insurance  rules  require 
that  "  Electric  gas  lighting  must  not  be  used  on  the  same  fixture 
with  the  electric  light.  The  above  rule  does  not  apply  to  frictional 
systems  of  gas  lighting."  It  is  better  to  have  the  gas  lighting  cir- 
cuit entirely  insulated  from  the  electric  lighting  circuit. 


FIG.  47.— HAND   DYNAMO   GAS   LIGHTER. 

46.     'How  many  jets  may  be  lighted  at  once  by  a  frictional  ma- 
chine f 

Machines  8  in.  in  diameter  will  light  from  one  to  thirty-five  burn- 


STATIC  ELECTRICITY.  11 

ers  at  once.  Some  machines  12  ins.  in  diameter  will  light  as  many 
as  one  hundred  jets  in  one  circuit. 

47.  Is  there  a  frictional  machine  for  lighting  single  jets? 
Such  are  sold  at  $3.50  to  $6.00,  having  in  the  handle  a  small  fric- 

tional generator  operated  by  the  thumb.  A  later  device  contains  a 
battery  which  heats  a  wire  to  incandescence. 

48.  How  much  do  the  frictional  machines  for  multiple  gas-light- 
ing cost? 

Frictional  electric  gas  lighting  machines  cost  from  $30  to  $70. 
The  multiple  jump-spark  burners  cost  25  to  50  cents  each. 

49.  Is  there  not  a  cheaper  method  of  lighting  gas  by  electricity? 
Yes.      When  single  jets,  or  groups  of  less  than  ten  are  to  be 

lighted,  it  is  cheaper  to  use  a  battery  with  a  "spark"  coil.  Some- 
times "induction  coils"  are  used  instead  of  frictional.  machines  for 
lighting  as  many  as  twenty-five  jets  at  once. 

50.  What  is  an  'induction  coil? 

It  consists  of  two  coils  of  wire  wound  around  a  bundle  of  iron 
wires.  One  coil  has  a  large  number  of  turns  of  fine  wire.  The 
other  coil  has  a  comparatively  small  number  of  turns  of  coarse 
wire  and  is  connected  to  a  battery  through  a  vibrator  or  interrupter 
that  closes  and  opens  the  circuit  rapidly,  so  as  to  make  the  current 


SPARK  T£RMWALS 


ITCH 


BATTER* 
.  TERMINALS 


FIG.  50.— INDUCTION  COIL. 

pulsate.  The  iron  core  is  magnetized  every  time  current  flows,  and 
loses  it  when  the  current  stops.  The  changing  magnetism  "induces" 
in  the  fine  wire  coil  a  small  current  at  very  high  pressure,  which 
will  jump  across  considerable  distances  and  may  be  used  for  multiple 
gas  lighting  or  other  purposes.  These  are  listed  at  from  $4  to  $75. 

51.  What  is  the  difference  between  an  induction  coil  and  a  medi- 
cal coil? 

A  medical  coil  is  a  variety  of  induction  coil  whose  secondary  or 


1$  ELECTRICAL   CATECHISM. 

high  pressure  winding  has  fewer  turns  of  wire  than  the  secondary 
of  an  induction  coil  intended  for  giving  sparks.  The  medical  coil, 
therefore,  gives  a  lower  pressure.  Such  coils  usually  have  some 
sort  of  regulating  device,  commonly  a  sleeve  or  tube  that  can  be 
slipped  over  the  iron  core.  The  currents  induced  in  the  tube  weaken 


FIG.    51.— MEDICAL    COIL. 

the  magnetic  induction  through  the  fine  wire  coil,  and  thus  reduce 
the  intensity  of  the  shock.  Electro-medical  treatment  should  be 
taken  with  great  caution,  as  harm  may  come  from  improper  use. 

52.     What  is  a  spark  coil? 

It  consists  of  a  single  coil  of  several  hundred  turns  of  wire  wound 
about  a  core  of  small  iron  wires,  which  become  magnetized  when  the 
circuit  is  closed,  so  that  current  from  the  battery  passes  through 


FIG.   52.— SPARK   COIL. 

the  coil.  When  the  circuit  is  opened,  the  core  loses  its  magnetism 
and  induces  a  high  pressure  in  the  coil,  so  that  a  strong  spark  ap- 
pears at  the  place  where  the  circuit  is  opened. 

53.     How  is  a  spark  coil  made  for  lighting  gas? 

For  the  core,  make  a  bundle  of  soft  iron  wires,  about  No.  19,  cut 
to  a  length  of  8  ins.  or  10  ins.  and  pulled  straight ;  make  the  bundle 
about  }  in.  in  diameter.  It  will  be  better  if  the  wires  are  shellacked 
or  varnished  before  being  tied  together.  Cover  the  core  with  sev- 
eral layers  of  dry  or  paraffined  paper ;  put  on  two  suitable  heads 
of  wood  or  fiber;  wind  on  about  seven  layers  of  No.  14  or  No.  18 


STATIC  ELECTRICITY. 


13 


annunciator  wire ;  bring  the  ends  out  to  suitable  binding  posts  on 
the  ends  of  the  spool,  and  cover  the  coil  with  binder's  cloth  or  with 
stiff  paper.  If  double  covered  magnet  wire  is  used  instead  of  the 
annunciator  wire,  the  whole  coil  should  be  boiled  in  paraffine  to 
improve  the  insulation. 

54.  What  kind  of  gas  jets  are  used  for  lighting  with  a  battery 
and  spark  coil? 

Several  varieties  are  used.     In  the  "  ratchet "  burner,  the  gas  is 


FIG.  54.— ELECTRIC  GAS  LIGHTING   BURNERS. 

turned  on  and  off  by  pulling  the  chain ;  the  same  motion  makes  the 
spring  wire  wipe  against  an  insulated  hook  near  the  gas  jet  and  thus 
closes  the  circuit ;  as  the  spring  snaps  past  and  breaks  the  circuit, 
the  spark  lights  the  gas.  In  the  "  stiff  pull  "  burner,  a  pull  turns 
the  gas  on  and  lights  it;  an  upward  push  turns  it  off.  To  increase 
the  life  of  the  battery,  the  best  lighters  make  a  spark  only 


FIG.  55.-AUTOMATIC   BURNERS. 


when  the  gas  -lights.  Some  cheaper  lighters  have  one  cock  for 
turning  the  gas  on  and  off,  and  a  separate  device  for  making  the 
spark. 


ELECTRICAL  CATECHISM. 


55.  What  are  automatic  electric  gas  burners? 

An  automatic  electric  gas  lighter  is  operated  by  an  electromagnet 
that  turns  on  the  gas  and  causes  sparks  near  the  tip.  In  some 
early  automatics,  the  same  magnet  shut  off  the  gas  when  the  key 
was  closed  long  enough.  Modern  automatics  have  two  magnets 
controlled  by  separate  keys.  The  gas  is  turned  on  and  lighted 
by  pressing  a  white  key,  or  is  turned  out  by  pressing  a  black 
key. 

56.  How  are  pendant  gas  burners  connected  to  the  circuit? 
One  terminal  of  the  battery  is  connected  to  the  gas-pipe  and  so  to 

the  gas  burner.  The  other  terminal  of  the  battery  is  connected 
through  the  spark  coil  to  an  insulated  wire  that  is  attached  to  the 
insulated  electrode  on  the  tip.  The  circuits  are  shown  in  the  ac- 
companying figure. 

57.  How  are  automatic  burners  connected? 

The  vibrating  arm  of  the  burner  and  one  terminal  of  the  battery 
are  electrically  connected  to  the  gaspipe  or  to  a  separate  insu- 
lated wire  (see  insurance  rule  quoted  in  No.  45).  The  insulated 
wire  from  the  other  terminal  of  the  battery  passes  through  the  spark 
coil  to  the  two  push-buttons,  whence  two  insulated  wires  go  to  the 
burner.  Here  one  circuit  passes  around  an  electromagnet  which 
turns  on  the  gas  and  ignites  it  by  means  of  the  spark  where  the  cir- 


cs» 


GAS  PIPE 


M« 


SPARKCOIU 


FIGS.  56-57-GAS-LIGHTING  CIRCUITS. 


cuit  is  broken  at  the  jet  The  other  circuit  passes  around  another 
electromagnet  which  turns  the  gas  off  when  the  dark  button  is 
pushed.  The  two  circuits  unite  beyond  the  coils  and  connect  with 
the  gas-pipe,  or  with  an  insulated  return  wire  to  the 


STATIC  ELECTRICITY.  15 

battery,  as  shown  in  the  figure.    For  convenience,  the  two  push- 
buttons are  white  and  black,  to  indicate  whether  the  room  will  be 


FIG.    57.-GAS-LIGHTING    PUSH-BUTTONS. 

lighted  or  darkened.  It  is  also  desirable  to  place  the  switch  so  that 
the  light  button  will  be  on  the  upper  side  and  the  dark  button  below, 
so  that  one  will  push  the  upper  button  to  turn  the  light  up  or  push 
the  lower  button  to  lower  the  light. 

58.  For  what  purposes  are  static  machines  used  other  than  for 
gas-lighting  f 

They  are  used  largely  for  medical  and  educational  purposes. 
Many  important  investigations  in  connection  with  lightning  arresters 
and  in  other  lines  have  been  carried  on  with  the  help  of  static  ma- 
chines in  laboratories  connected  with  electrical  manufactories  and 
colleges. 

59.  Is  the  electricity  on  belts  caused  by  friction  f 

It  is  caused  partly  by  internal  friction  in  the  belt  as  it  bends 
around  the  pulley ;  partly  by  friction  between  the  belt  and  the  pulley ; 
partly  by  friction  between  the  belt  and  the  air ;  largely  by  induction 
as  the  charges  are  separated  by  the  motion  of  belt. 

60.  Why  does  the  belt  give  more  electricity  on  some  days  than 
on  others? 

When  the  air  is  moist  it  is  not  so  good  an  insulator.  The' belt 
also  becomes  more  or  less  damp  and  allows  the  electricity  to  pass 
along  to  the  pulley  and  escape. 

61.  How  can  a  person  tell  whether  a  belt  is  charged  with  elec- 
tricity? 

A  spark  or  bluish  colored  brush  will  pass  from  the  belt  to  one's 


16  ELECTRICAL   CATECHISM. 

hand  or  to  any  piece  of  metal  held  near,  if  the  belt  is  charged.    If  one 
stands  near  a  belt  that  is  highly  charged,  the  electricity  will  cause 


FIG.  61.— ELECTRICITY  FROM   BELT. 

his  hair  to  stand  out  and  he  will  feel  a  peculiar  sensation,  as  if  he  had 
brushed  against  cobwebs. 

62.  Is  the  electricity  from  a  belt  dangerous? 

One  may  sometimes  obtain  an  uncomfortable  shock,  but  it  is  not 
considered  dangerous  to  persons.  It  may  cause  trouble  to  electrical 
dynamos  and  motors. 

63.  Why  is   the   belt  electricity   dangerous   to   dynamos    and 
motors? 

Because  it  sometimes  jumps  through  the  insulation,  causing  a 
spark.  The  regular  current  from  the  machine  is  not  ordinarily  of 
high  enough  pressure  to  jump  through  insulation,  but  it  can  follow 
a  spark.  Sometimes  the  static  electricity  from  the  belt  charges  the 
iron  frame  of  the  machine,  jumps  through  the  insulation  to  the  wires, 
and  so  "grounds"  the  machine.  On  arc  light  or  alternating-current 
machines  using  high  potentials,  this  makes  it  dangerous  to  touch 
even  the  frame  when  the  machine  is  running.  Under  some  cir- 
cumstances this  may  cause  the  machine  to  become  short-circuited, 
and  so  to  burn  out. 

64.  Is  electricity  from  belts  liable  to  cause  fires? 

A  number  of  cases  are  known  where  fires  were  started  in  this 
way.  The  sparks  have  set  fire  to  the  vapors  of  benzine  or  gasoline 
used  for  cleaning  type  in  printing  offices,  or  for  cleaning  clothes  in 
tailor  shops.  Such  sparks  might  also  set  fire  to  oily  waste  or  other 
inflammable  material. 

65.  Are  fires  caused  by  frictional  electricity  other  than  that  from 
belts? 

Several  fires  have  occurred  in  dressmaking  establishments  from 
sparks  caused  by  friction  when  cleaning  dresses  with  benzine. 

66.  What  causes  the  peculiar  smell  like  wet  matches  often  no- 
ticed near  belts  that  are  charged  with  static  electricity? 

The  smell  is  that  of  the  ozone  that  is  formed  from  the  oxygen  of 
the  air  by  the  electric  discharge. 


STATIC  ELECTRICITY.  17 

67.  Is  this  smell  dangerous  or  poisonous? 

It  is  not  poisonous,  but  healthful.  It  is  believed  to  be  a  source 
of  danger,  since  oily  waste  and  similar  substances  will  ignite  spon- 
taneously more  easily  if  much  ozone  is  present.  Fires  once  started 
will  spread  faster  when  the  air  contains  ozone. 

68.  Can  any  practical  use  be  made  of  this  ozone? 

It  helps  make  the  engine  room  or  central  station  a  healthy  place 
to  work.  No  commercial  use  seems  to  have  been  made  of  the  ozone 
developed  by  belts,  although  ozone  is  frequently  made  in  special 
apparatus  for  medical  purposes.  Ozone  is  also  used  for  bleaching, 
disinfecting,  drying  and  seasoning,  for  purifying  water  and  for  other 
purposes  where  a  strong  oxidizing  agent  is  desired. 

69.  How  is  ozone  made? 

An  ozone  generator  usually  consists  of  two  conducting  surfaces, 
which  are  well  insulated  from  each  other  and  placed  at  such  a  dis- 
tance apart  that  when  oppositely  charged  there  will  be  a  "silent  dis- 
charge" between  them  as  the  electricity  slowly  passes  from  one  to 
the  other  across  the  intervening  air.  When  air  passes  between  such 
surfaces  the  oxygen  of  the  air  (whose  molecules  each  consist  of  two 
atoms),  is  dissociated  and  recombined  into  ozone,  each  molecule  of 
which  contains  three  atoms.  The  ozone  has  a  strong  tendency  to 
break  up,  and  the  atoms  thus  set  free  unite  with  or  "oxidize"  other 
substances.  The  exact  process  by  which  the  oxygen  is  changed  into 
ozone  by  the  action  of  electricity  is  not  completely  understood. 

70.  What  are  electric  belts? 

The  expression  is  used  in  three  distinct  senses.  First,  in  refer- 
ence to  belts  that  develop  considerable  quantities  of  static  electricity, 
as  considered  in  Nos.  59  to  66  above.  Secondly,  it  is  sometimes 
used  as  a  name  for  special  brands  of  belting  of  selected  material  of 
uniform  weight,  width  and  thickness,  which  have  been  developed 
for  operating  electric  or  other  fast  running  machinery.  Thirdly, 
the  term  is  used  for  devices  intended  for  wearing  on  the  human  body 
and  supposed  to  cure  all  manner  of  weakness  and  disease. 

71.  Are  electric  belts  of  any  real  medicinal  value? 
Manufacturers  and  vendors  of  electric  belts  have  many  authentic 

testimonials  of  undoubted  cures  that  have  followed  the  use  of  elec- 
tric belts.  In  most  such  cases  the  cures  are  due  to  the  expectation 
of  the  wearers  and  to  that  extent  only  are  due  to  electricity.  The 
great  success  of  such  articles  for  the  most  part  simply  illustrates  the 
immense  influence  of  the  mind  over  the  body;  and  a  red  string  tied 


18  ELECTRICAL   CATECHISM. 

to  a  shark's  tooth  would  do  equally  well,  could  it  be  associated  with 
electricity  in  the  mind  of  the  wearer.  A  few  belts  really  yield  elec- 
tricity. 

72.  What  are  electric  castors? 

These  are  castors  with  wheels  made  of  glass  or  other  insulating 
material.  They  are  supposed  to  keep  the  electricity  of  the  body 
from  escaping  to  the  earth.  Therefore,  rf  one  sleeps  on  a  bed 
with  glass  or  porcelain  castors,  or  on  one  with  the  feet  set  on  glass 
knobs,  he  will  sleep  better  and  will  get  well.  The  recovery  comes 
from  the  better  sleep,  which  is  a  result  of  the  confidence  in  the 
effectiveness  of  the  "invaluable"  device.  If  one  pays  a  good  round 
sum  for  such  devices,  he  believes  they  will  help  him;  they  do  help, 
if  he  has  sufficient  faith. 

73.  Are  electric  belts  and  castors  to  be  commended? 

They  certainly  will  not  harm  anything  except  the  pocketbook,  and 
to  that  extent  are  preferable  to  drugs  and  patent  medicines. 

74.  Why  do  printers  object  to  static  electricity? 

When  the  paper  is  electrified,  the  sheets  stick  together  so  that  it  is 
more  difficult  to  separate  them  in  feeding  to  the  press ;  they  stick  to 
parts  of  the  press,  so  that  it  sometimes  requires  two  men  to  feed  the 
paper  to  the  press ;  sometimes  the  paper  will  stick  to  the  cylinder  or 
blanket  so  tightly  that  it  is  torn  in  removal ;  in  cylinder  presses,  the 
paper  will  sometimes  stick  to  the  fly  and  necessitate  a  stop  with  loss 
of  time ;  it  does  not  deliver  regularly  and  will  not  pile  evenly,  so  that 
much  time  and  stock  are  lost  in  preparing  the  work  for  folding  or 
cutting ;  the  sheets  stick  so  tightly  together  that  the  fresh  ink  offsets 
and  smuts ;  the  presses  must  be  run  slower  to  avoid  trouble ;  some- 
times the  pressmen  receive  severe,  or  at  least,  annoying  shocks. 

75.  What  causes  the  static  electricity  about  printing  presses? 
Part  of  it  comes  with  the  paper,  which  becomes  highly  electrified 

during  the  process  of  calendering  to  such  an  extent  that  sparks  6  ins. 
or  8  ins.  sometimes  jump  from  the  paper,  such  electrification  making 
it  uncomfortable  for  the  workmen.  Some  of  this  electricity  remains 
on  the  paper  when  it  is  packed,  the  amount  varying  with  the  quality 
of  paper,  the  treatment  it  receives  at  the  mill  and  the  dryness  of  the 
warehouse.  The  paper  also  becomes  electrified  in  the  printing  office, 
as  the  sheets  are  drawn  over  one  another  in  feeding  the  press  or  in 
other  handling.  The  press  seems  to  become  electrified,  especially 
when  the  ink  is  sticky,  the  electrification  seeming  to  be  due  partly 
to  the  tearing  apart  of  the  ink.  Probably  some  of  the  electricity  is 


STATIC  ELECTRICITY.  19 

due  to  the  bending  and  stretching  of  the  paper  under  the  type  and 
against  the  blanket. 

76.  How  can  static  electricity  be  removed  from  a  printing  office? 
A  reasonable  amount  of  moisture  in  the  air  is  the  most  common 

cure.  The  moisture  from  the  air  makes  everything  slightly  damp,  and 
water  is  a  good  conductor  fbr  static  electricity.  The  result  is  that 
the  positive  and  negative  charges  come  together  and  become  neutral- 
ized, much  of  it  going  to  the  earth.  Too  much  moisture,  however, 
is  apt  to  cause  trouble  by  rusting.  A  common  method  is  to  "ground" 
the  presses  and  also  to  provide  special  conductors  for  taking  the 
static  electricity  from  the  paper  to  the  earth.  Connection  with  the 
water  or  gas-pipes  will  usually  give  sufficient  connection  with  the 
earth,  although  it  is  better  in  some  cases  to  connect  with  a  pipe  or  rod 
driven  into  the  earth  to  a  depth  where  it  is  permanently  damp. 

77.  How  is  steam  applied  for  removing  the  static  electricity? 
Where  the  paper  is  fed  from  a  roll  to  a  cylinder  press,  steam  is 

sprayed  directly  upon  the  paper  through  fine  holes  in  a  steam  pipe 
close  to  the  press  where  the  paper  enters.  In  other  cases  the  steam 


STMf 


FIG.  77.— STEAM  SPRAYER. 

is  diffused  so  as  to  form  a  sort  of  mist  or  small  cloud  of  vapor.  The 
sketch  shows  one  method  of  applying  from  a  pipe  6  ins.  below  the 
tapes  of  the  delivery  of  a  cylinder  press  so  that  the  paper  is  moistened 
as  it  leaves  the  cylinder. 

78.  How  can  the  electricity  be  removed  when  it  is  desirable  not 
to  moisten  the  paper? 

A  brush  of  fine  wires  arranged  to  trail  upon  the  paper  will  collect 
much  of  the  charge,  and  if  well  connected  with  the  earth  will  carry 
it  off.  Hot  air  has  been  applied  successfully  for  this  purpose.  The 
paper  may  also  be  passed  between  "grounded"  metallic  rollers,  which 


20 


ELECTRICAL   CATECHISM. 


will  take  off  any  charge  that  may  be  on  the  paper  as  it  enters  the 
press. 

79.  How  can  hot  air  be  used  to  remove  static  electricity? 

Very  hot  air  is  a  fair  conductor  for  electricity.  Any  current  of  air 
will  dissipate  a  charge,  since  each  particle  of  air  coming  in  contact 
with  the  charge  takes  up  a  small  part  of  the  charge  and  carries  it 
away.  A  method  in  successful  use  is  to  place  under  the  feed-board 
of  the  cylinder  press  a  gas-pipe  in  which  have  been  drilled  a  number 
of  holes  one  and  a  half  inches  apart,  and  large  enough  to  give  gas 
flames  about  an  inch  high.  The  current  of  warm  air  arising  from 
this  will  dissipate  the  electricity. 

80.  Why  is  the  electricity  worse  on  a  cold  day? 

Probably  because  there  is  not  so  much  moisture  in  the  air  when 
it  is  cold.  Consequently  everything  is  dryer  and  is  less  able  to  con- 
duct the  charges  away.  Cold  air  is  a  better  insulator  than  hot. 

81.  How  can  electricity  be  removed  from  belts? 

A  common  method  is  to  fasten  near  the  belt  a  number  of  sharp 
metallic  points  which  are  connected  with  the  ground.  One  way  of 
doing  this  is  to  drive  a  number  of  sharp  tacks  through  a  piece  of  tin, 
or  to  solder  some  small  wires  to  a  large  one,  and  to  fasten  the  row 
of  points  near  the  pulley  so  as  to  be  within  an  inch  of  the  belt.  An- 


FIG.  81.-STATIC  COLLECTORS. 

other  method  is  to  stretch  a  wire  lengthwise  of  the  belt  and  a  few 
inches  above  it.  Still  another  plan  is  to  fasten  a  bundle  of  fine  wires, 
such  as  are  used  in  lamp  cord,  a  short  distance  above  the  belt.  Any 
of  these  devices  will  take  most  of  the  electricity  from  the  belt,  if 
electrically  connected  with  the  ground  and  if  the  points  are  close 
enough  to  the  belt.  Of  course,  care  must  be  taken  that  they  do  not 
become  caught  in  the  belt.  (See  also  Nos.  106  to  109.) 


STATIC  ELECTRICITY. 


21 


82.  What  methods  are  used  for  taking  the  static  electricity  away 
from  shafting  and  machines? 

The  best  way  is  to  "ground"  the  hangers  or  some  other  stationary 
part  by  means  of  a  wire  fastened  to  a  water,  gas  or  steam  pipe,  or  to 
a  rod  driven  into  damp  earth. 

83.  Is  this  method  used  with  dynamos  and  motors? 

Not  generally.  The  frames  of  such  machines  are  usually  insulated 
from  the  ground  so  as  to  reduce  the  danger  of  the  circuits  getting 
"grounded"  and  so  allowing  the  current  to  escape  or  cause  damage. 

84.  How  may  the  static  electricity  be  removed  from  dynamos  or 
motors  without  grounding  them? 

The  static  electricity  is  of  so  high  pressure  that  it  will  travel  over 
paths  that  are  good  insulators  for  the  regular  current.  Such  a  path 
is  furnished  by  a  moistened  string,  a  pencil  mark  on  a  piece  of  paper, 
or  a  can  of  pure  water.  Place  a  can  of  water  on  the  floor  under  or 
near  the  machine,  connect  it  to  a  water  or  gas-pipe  by  means  of  a 
small  wire,  tie  a  string  to  some  convenient  part  of  the  machine,  and 


FIG.  84.— REMOVING  STATIC  ELECTRICITY. 

let  the  end  of  the  string  dip  into  the  water.  The  string  will  soon 
become  moist  enough  to  carry  off  the  static  charge  without  ground- 
ing the  machine,  Fig.  84.  A  neater  and  cleaner  way  is  to  make  a 
long  mark  with  a  common  lead  pencil  upon  a  narrow  strip  of  paper ; 
fasten  one  end  of  this  under  the  head  of  a  bolt  or  other  convenient 
place  so  that  one  end  of  the  pencil  mark  touches  the  machine;  a 
small  wire  attached  to  a  good  ground  may  be  connected  with  the 
other  end  of  the  pencil  mark  by  means  of  a  washer  and  screw. 

85.  Why  do  some  belts  give  more  electricity  than  others? 
Rubber  belts  are  usually  more  highly  charged  than  leather  belts 

because  they  do  not  take  up  moisture  and  are  better  insulators. 

86.  Can  any  practical  use  be  made  of  the  electricity  from  belts? 
It  might  possibly  be  used  for  making  ozone,  ammonia  or  nitric  acid 


22  ELECTRICAL  CATECHISM. 

from  the  air.  So  far  as  is  known,  it  has  not  been  so  used  to  any 
great  extent.  Much  can  be  learned  by  experimenting  with  it. 

87.  What  experiments  can  be  made  with  static  electricity? 
Instructive  and  interesting  experiments  may  be  made  with  simple 

apparatus  to  show  attraction  and  repulsion,  conduction,  induction, 
the  effect  of  points,  the  action  of  the  condenser  and  the  magnetic, 
heating  and  physiological  effects  of  electricity. 

88.  How  can  attraction  and  repulsion  be  shown  by  electricity 
from  a  beltf 

Hold  a  doll's  head  near  a  belt,  and  the  hair  will  be  attracted  to  it. 
Each  hair  will  stand  on  end  as  if  attracted  by  the  belt,  and  also  stand 
apart  as  if  repelled  by  all  the  other  hairs.  A  similar  effect  occurs  if 
one  places  his  own  head  near  the  belt,  being  careful,  of  course,  not  to 
get  caught  by  it,  A  miniature  hailstorm  or  a  set  of  dancing  dolls 
can  easily  be  arranged. 

89.  How  can  dolls  be  made  to  dance  by  electricity? 

Small  dolls  or  other  images,  cut  from  dry  pith  of  the  elder  or  corn- 
stalk, are  placed  on  a  metallic  pan  or  plate  that  is  connected  with  the 
ground.  Directly  above  them  is  suspended  a  similar  plate  that  is  well 
insulated  from  the  ground,  but  is  connected  with  a  wire  or  brush  near 
the  belt.  If  the  belt  is  giving  off  electricity,  the  upper  plate  becomes 
charged  and  attracts  the  pith  dolls.  As  soon  as  they  touch  it  they 
become  charged  with  the  same  kind  of  electricity  as  the  upper  plate, 


FIG.  89.— ELECTRIC  DOLLS. 

and  are  at  once  repelled  (or  as  seems  more  likely,  are  no  longer  at- 
tracted), and  fall  to  the  lower  plate.  Here  the  charge  they  received 
from  the  upper  plate  goes  off  to  the  ground  and  they  are  again  at- 
tracted by  the  upper  plate.  In  this  way  they  will  dance  up  and  down 
in  a  very  amusing  fashion.  Instead  of  the  pith  images,  bits  of  straw, 
grass  or  paper  may  be  used  to  make  a  miniature  hailstorm. 


STATIC  ELECTRICITY.  23 

90.  Is  electrostatic  repulsion  of  any  practical  importance? 

It  is  frequently  troublesome,  as  when  one  undertakes  to  arrange 
his  hair  with  a  rubber  comb  on  a  dry  day.  The  electrification  of  the 
hairs  causes  them  apparently  to  repel  one  another  and  refuse  to  be 
placed.  It  has  been  proposed  to  throw  off  disease  germs  and  other 
dirt  by  the  application  of  electric  charges  at  high  potential.  Actual 
use  is  made  of  such  action  in  the  automatic  drying  of  the  insulators 
used  on  lines  for  transmitting  electric  power  at  high  potentials,  the 
particles  of  moisture  being  actually  driven  off,  so  that  the  insula- 
tors become  dry  even  on  a  misty  day.  When  the  electrical  pressure 
is  much  above  30,000  volts,  there  is  quite  an  appreciable  loss  of 
energy  from  the  charges  carried  off  by  the  air  passing  the  wires. 
Electrostatic  attraction  and  repulsion  is  a  source  of  much  trouble  in 
printing  offices  (see  Nos.  74  and  80),  and  in  woolen  and  cotton  mills. 

91.  Is  electrostatic  repulsion  used  in  telegraphy? 

The  "siphon  recorders"  used  in  connection  with  submarine  cables 
were  based  on  this  phenomenon.  The  current  is  too  weak  to 
operate  any  ordinary  relay  or  recording  device  requiring  appreciable 


FIG.    91.— SIPHON    RECORDER. 


force.  To  secure  a  record  without  friction,  a  fine  tube,  D,  connecting 
with  an  inkwell,  B,  was  attached  to  the  coil  of  a  delicate  galvanometer, 
and  also  to  one  terminal  of  an  electrostatic  generator,  M,  so  that  a 
stream  of  fine  drops  of  ink  was  thrown  out  by  repulsion  and  so  records 
motions  of  tube  past  a  paper  ribbon.  A  magnet  now  jolts  the  siphon. 


24  ELECTRICAL  CATECHISM. 

92.  How  is  static  electricity  troublesome  in  spinning  and  weav- 
ing? 

The  fibres  become  electrified  and  refuse  to  lie  close  together,  caus- 
ing the  threads  and  fabric  to  become  loose.  To  overcome  the  trouble, 
it  is  common  to  keep  the  air  moist  and  to  connect  the  machines  with 
the  earth,  so  as  to  dissipate  or  carry  off  whatever  static  electricity  ap- 
pears. Somewhat  similar  trouble  is  experienced  in  paper  mills. 

One  of  the  advantages  of  the  "indirect  radiation"  system  of  heat- 
ing and  ventilation  is  that  the  humidity  of  the  air  is  under  easy  con- 
trol. 

93.  Is  static  electricity  objectionable  around  electric  light  plants? 
It  is,  for  it  frequently  causes  the  machines  to  break  down  and  more 

frequently  breaks  incandescent  lamps.  Smaller  amounts  of  static 
charge  often  get  in  electrical  measuring  instruments  and  cause  the 
pointer  to  stick  or  to  give  incorrect  readings. 

94.  How  does  static  electricity  break  down  machines  f 

The  static  electricity  is  liable  to  jump  from  the  wires  to  the  ground 
or  to  the  iron  frame  of  the  machine  in  two  places  at  once.  ( See  No. 
132.)  The  current  from  the  line  can  follow  the  path  of  the  spark, 
although  it  could  not  jump  and  start  the  spark.  This  current  burns 
the  insulation,  and  unless  immediately  stopped  will  melt  the  wires. 

95.  How  does  static  electricity  break  incandescent  lamps? 

The  sparks  pierce  the  glass  globe  of  the  lamp  in  jumping  to  or 
from  the  filament.  This  allows  air  to  enter  and  destroy  the  vacuum, 
the  filament  being  burned  as  soon  as  sufficient  air  enters.  (See  No. 
125).  More  frequently  the  static  charge  on  the  globe  attracts  the 
filament  until  it  touches  the  glass,  and  the  intense  heat  then  cracks 
the  glass  so  that  air  enters. 

96.  Has  any  commercial  use  been  made  of  electrostatic  attrac- 
tion? 

It  is  used  to  some  extent  in  picking  up  labels  in  connection  with 
pasting  machines. 

97.  Is  electrostatic  attraction  and  repulsion  of  any  importance  in 
nature? 

Fog  and  mist  are  cleared  up  in  many  cases  by  electrical  action. 
The  heavy  downfall  of  large  drops  of  rain  immediately  following 
lightning  discharges  during  thunder  showers  is  attributed  to  similar 
action.  When  the  air  is  unevenly  electrified,  the  opposite  sides  of 
the  small  drops  of  water  forming  the  mist  or  fog  or  raincloud  are 
unequally  electrified  and  attract  one  another,  thus  forming  larger 


STATIC  ELECTRICITY.  25 

drops  which  have  less  surface  than  the  added  surfaces  of  the  in- 
dividual drops,  and  consequently  fall  to  the  earth.  When  the  air  is 
evenly  electrified,  the  water  particles  repel  one  another  and  remain 
small,  so  that  the  mist  or  fog  continues. 

98.     Can  electric  bells  be  rung  by  static  electricity? 

Not  ordinarily.  Telephone  bells  will  sometimes  give  a  single 
stroke  when  the  line  becomes  charged  by  lightning.  A  special  kind 
of  bell  known  as  the  "electric  chime"  may  be  operated  by  static  elec- 
tricity from  a  belt  or  other  source.  Two  or  more  bells  are  suspended 
at  the  same  height  and  about  a  half-inch  apart,  as  indicated  by  the 
sketch.  Alternate  bells  are  connected  with  the  ground.  The  remain- 
ing bells  are  well  insulated  and  are  connected  with  a  wire  or  brush 
that  collects  electricity  from  the  belt.  A  light  metallic  ball  is  sus- 
pended between  each  pair  of  bells  by  means  of  a  silk  thread.  Each 
ball  will  be  attracted  by  the  bell  that  is  charged  from  the  belt.  After 


FIG.  98.-ELECTRIC  CHIME. 

touching  that  bell,  it  becomes  charged  with  the  same  kind  of  elec- 
tricity and  then  is  no  longer  attracted  by  that  bell,  but  is  attracted  by 
the  other  bell  that  is  grounded.  When  it  strikes  the  grounded  bell, 
it  loses  its  charge  and  is  again  attracted  to  the  first  bell.  Thus  it 
swings  back  and  forth,  ringing  both  bells  continuously.  The  elec- 
tric chime  was  m  vented  by  Benjamin  Franklin  in  1752,  to  indicate 
the  presence  of  thunderclouds. 

99.     What  is  meant  by  the  induction  of  static  electricity? 

Hang  a  tin  pail  near  a  belt.  Several  inches  from  the  first  pail  hang 
a  second  pail  that  is  connected  with  a  third  one  by  a  wire,  as  sug- 
gested in  the  figure.  All  of  the  pails  should  be  suspended  by  dry 
cotton  or  silk  thread,  or  should  be  placed  on  dry  glass  jars  or  bot- 
tles, so  as  to  be  well  insulated  from  the  ground.  When  the  first  pail 
is  well  charged  with  electricity  from  the  belt  one  may  get  a  shock 


26  ELECTRICAL   CATECHISM. 

by  touching  the  third  one.  The  pith  ball  or  leather  electroscope 
will  also  show  that  the  third  one  is  charged.  Since  the  second  and 
third  pails  are  well  insulated  from  the  first  and  from  the  belt,  and 
since  they  are  too  far  apart  for  the  electricity  to  pass  through  the 


FIG.  99.-ELECTROSTATIC  INDUCTION. 

air  to  them  (according  to  the  common  theory,  which  does  not  quite 
agree  with  the  explanations  of  the  more  modern  "displacement" 
theory),  the  electricity  does  not  seem  to  get  into  them  by  conduction, 
but  by  some  other  process.  This  is  explained  by  the  principle  of  in- 
duction. The  first  pail  is  given  a  positive  charge  by  the  belt.  Ac- 
cording to  the  old  two-fluid  theory,  the  positive  charge  attracts  the 
negative  electricity  in  the  other  pails  and  repels  their  positive  elec- 
tricity. The  negative  electricity  of  the  second  and  third  pails  gets 
as  close  as  possible  to  the  positively  charged  first  pail,  that  is,  it  goes 
to  the  near  side  of  the  second  pail.  The  positive  gets  as  far  away  as 
possible  and  is  found  to  be  in  the  further  or  third  pail,  and  it  leaves 
that  also  when  the  hand  or  any  other  conductor  furnishes  a  path 
for  it  to  go  still  further  away. 

100.  Is  electrostatic  induction  of  much  importance? 

The  strongest  machines  for  developing  static  electricity  are  based 
on  induction.  Most  of  the  so-called  lightning  on  electric  wires  is 
induced  rather  than  directly  drawn  from  the  clouds  or  lightning. 
Most  of  the  "cross-talk"  and  the  confusing  noises  on  telephone  lines 
come  from  electrostatic  induction. 

101.  Explain  how  cross-talk  on  telephone  lines  is  caused  by  in- 
duction? 

The  currents  used  in  telephony  are  alternating ;  that  is,  each  wire 
is  alternately  positive  and  negative,  changing  polarity  many  times 
each  second.  The  electrostatic  induction  between  neighboring  wires 
is  most  easily  explained  by  some  experiments  made  by  Carty.  Sup- 
pose that  one  side  of  a  telephone  circuit  is  near  one  wire  of  an- 
other circuit,  as  shown  in  the  figure.  At  an  instant  when  the  dis- 
turbing wire  is  positive,  a  negative  charge  is  induced  in  the  near  side 


STATIC  ELECTRICITY.  27 

of  the  telephone  circuit,  and  the  positive  charge  escapes  to  the  fur- 
ther side.  The  rearrangement  of  the  charges  causes  a  momentary 
current  as  the  positive  electricity  passes  to  the  further  conductor,  a 
result  being  that  the  current  causes  noise  in  the  telephones  at  the 


FIG.   101.— ELECTROSTATIC   INDUCTION. 

ends.  Carty  found  that  the  noise  was  not  affected  if  the  wires  were 
cut  at  the  points  marked  a  and  b,  and  also  that  a  telephone  would 
give  no  noise  or  evidence  of  current  when  connected  into  the  circuit 
at  a  or  b,  but  would  when  connected  between  a  and  b.  These  experi- 
ments showed  that  the  current  causing  noise  in  the  telephones  at  the 
ends  was  not  caused  by  electromagnetic  induction,  since  such  cur- 
rents require  a  completely  closed  circuit.  When  the  disturbing  wire 
carries  an  alternating  current  or  fluctuating  current,  such  as  used 
for  electric  lighting,  the  potential  continually  changes,  causing  cor- 
responding changes  in  neighboring  circuits,  and  thus  causing  noises 
in  telephones.  If  the  distributing  wire  carries  a  telephone  current, 
such  as  caused  by  talking,  the  induced  currents  are  similar,  and  the 
talk  is  heard  in  neighboring  circuits.  It  should  be  remarked  that 
much  of  the  cross-talk  is  due  to  leakage  caused  by  imperfect  in- 
sulation. 

102.    How  can  cross-talk  be  avoided? 

The  part  caused  by  leakage  is  reduced  by  improving  the  insula- 
tion. That  caused  by  electrostatic  induction  may  be  reduced  by  in- 
creasing the  distance  between  the  two  circuits.  Since  the  latter  is 


DISTURBING  WIRt 


FIG.  103.-TRANSPOSITION  TO  NEUTRALIZE  INDUCTION. 

not  practicable  for  many  circuits,  the  wires  are  transposed  at  certain 
intervals,  so  that  the  induction  in  one  section  balances  that  in  another,, 

103.     Explain  how  transposition  neutralises  cross-talk. 

Suppose  that  the  disturbed  wires  are  crossed,  as  suggested  in  the 
figure.  The  negative  charges  at  A  and  B  have  four  paths  to  the 
opposite  wire,  and  since  the  resistance  of  the  paths  at  the  crossing 
is  much  less  than  that  through  the  instruments  at  the  ends,  most  of 


28 


ELECTRICAL   CATECHISM. 


the  induced  current  passes  through  the  crossing  wires,  thus  reducing 
the  current  and  the  noise  in  the  telephones.  The  practice  on 
actual  telephone  lines  is  to  transpose  the  wires  every  quarter  or  half 
mile.  When  a  number  of  circuits  are  carried  on  the  same  poles,  it 
is  a  matter  requiring  much  experience  and  judgment  to  arrange  the 
transpositions  so  that  no  circuit  shall  affect  any  other. 

104.     What  effect  do  points  have  upon  static  electricity? 
They  scatter  it.     An  interesting  experiment  is  to  connect  a  pointed 
wire  with  a  spark  collector  and  bring  the  point  near  to  a  candle,  as 


FIG.    104.— ELECTRIC    BREEZE. 

suggested  in  the  figure.  A  sort  of  wind  seems  to  come  from  the 
point  and  blows  the  flame  to  one  side,  or  may  even  extinguish  it.  A 
similar  thing  occurs  in  the  electric  whirligig. 

105.     What  is  an  electric  whirligig? 

It  consists  of  a  number  of  wires  radiating  from  a  common  center 
and  resting  upon  a  metallic  point  upon  which  they  may  rotate  with 
but  little  friction.  The  outer  ends  of  the  wires  are  pointed  and  are 


FIG.    105.— ELECTRIC    WHIRLIGIGS. 


bent  backward  like  the  arms  of  a  lawn  sprinkler,  as  in  the  figure. 
One  may  be  made  by  soldering  several  wires  of  the  same  length  to  a 
small  tin  disc,  the  center  of  which  has  been  slightly  raised  by  a  center 


STATIC  ELECTRICITY. 


29 


punch,  so  as  to  form  a  socket  or  bearing.  Balance  this  upon  the 
pointed  end  of  a  stiff  wire  or  needle  that  is  well  insulated  from  the 
ground.  Connect  the  upright  wire  or  needle  to  the  spark  collector  or 
to  a  wire  near  the  belt.  If  the  whirligig  is  nicely  balanced  and  the 
standard  is  well  insulated,  it  will  rotate  as  long  as  the  belt  is  giving 
off  electricity  to  the  wire. 

106.  How  is  this  effect  of  sharp  points  explained? 

The  reason  generally  given  is  that  static  electricity  stays  on  the 
outside  of  conductors  or  in  the  insulating  medium  around  them. 
Where  the  conductor  is  small,  the  charge  seems  to  pile  up,  its  density 
being  inversely  proportional  to  the  radius  of  curvature  of  the  sur- 
face. At  sharp  edges  or  points,  the  charge  is  stronger  or  denser 
than  at  other  parts  of  a  conductor.  Since  electricity  of  the  same 
kind  repels  itself,  it  tends  to  scatter  or  discharge  at  such  places. 
When  the  insulating  material  (often  called  the  dielectric)  is  air,  oil 
or  other  fluid,  the  dielectric  itself  is  repelled  and  a  current  is  set  up, 
each  particle  of  air  or  oil  carrying  away  some  of  the  charge.  This  is 
nicely  illustrated  by  the  candle  and  the  whirligig.  The  current  of 
electrified  air  from  the  point  blows  the  candle  flame.  The  charge 
upon  the  point  is  also  repelled  as  strongly,  and  there  is  a  backward 
pressure  upon  the  wire  exactly  equal  to  that  of  the  current  of  air. 
If  the  point  is  free  it  will  move  backward,  as  shown  by  the  whirligig, 
which  is  something  like  a  turbine  wheel. 

107.  Is  any  practical  use  made  of  the  action  of  points  in  scatter- 
ing an  electric  charge? 

Most  forms  of  lightning  arrester   use  points.     Telegraph   pro- 


F1G.  107.— LIGHTNING   ARRESTERS. 


tectors  frequently  have  serrated  brass  plates,  the  middle  one  being 
grounded.     Telephone  protectors  generally  use  carbon  plates  with 


30  ELECTRICAL  CATECHISM. 

mica  separators,  the  rough  carbon  surfaces  presenting  a  multitude 
of  fine  points  across  which  charges  are  dissipated  (see  156).  Barbed 
wires  are  sometimes  strung  over  power  circuits. 

1 08.  Do  points  collect  or  attract  electricity  as  well  as  they  scat- 
ter it? 

It  is  a  common  but  erroneous  idea  that  the  points  on  a  lightning 
rod,  or  an  arrester,  or  belt  discharger,  actually  collect  the  electricity 
from  the  clouds  or  from  the  belt.  The  explanation  preferred  by 
scientists  is  that  the  charge  in  the  clouds  or  on  the  belt  induces  an 
opposite  charge  on  the  pointed  metal  as  explained  in  answer  to  No. 
99.  Suppose  the  belt  is  charged  positively :  electricity  is  induced  in 
the  arrester,  the  positive  part  being  repelled  and  going  off  to  the 
earth ;  the  negative  part  is  attracted  to  the  points  and  is  there  scat- 
tered, as  explained  in  No.  106.  As  the  negative  charge  escapes  from 
the  points,  it  unites  with  and  neutralizes  the  positive  charge  on  the 
surface  of  the  belt  or  in  the  cloud.  While,  therefore,  positive  elec- 
tricity flows  away  from  the  lightning  or  spark  arrester,  it  is  the  in- 
duced positive  electricity  and  not  that  originally  in  the  cloud  or  on 
the  belt. 

109.  What  is  the  reason  why  a  bundle  of  fine  wires  discharges  a 
belt  better  than  a  single  large  wire? 

This  is  for  the  same  reason  as  above.  The  bundle  of  fine  wires 
presents  many  sharp  points,  each  of  which  helps  to  neutralize  the 
charge.  For  the  same  reason  also,  a  piece  of  tin  with  a  rough  edge 
or  with  many  sharp  points  is  a  good  spark  arrester  or  discharger. 

no.  Does  Nature  make  any  use  of  the  electrical  properties  of 
sharp  points? 

It  is  known  that  static  electricity  has  much  to  do  with  the  growth 
of  plants.  Many  leaves  have  sharp  points  on  the  spines  along  the 
edges,  while  others  are  covered  with  finely  pointed  hairs.  It  is  be- 
lieved that  these  fine  points  are  used  in  collecting  electricity  for  the 
growth  of  the  plant.  The  fact  that  grass  grows  but  sparingly  under 
cedar  and  pine  trees  is  largely  due  to  the  fact  that  the  many  sharp 
points  on  the  pine  needles  take  nearly  all  the  electricity  out  of  the 
air,  so  that  little  is  left  for  the  growth  of  other  vegetation  beneath. 
(See  also  Nos.  108  and  154.) 

in.     What  is  a  Ley  den  jar? 

This  can  be  understood  best  from  an  experiment.  Partially  fill 
a  bottle  with  water,  carefully  dry  the  outside  and  place  in  the  bottle  a 
wire  long  enough  to  extend  several  inches  above  the  top.  Hold  the 
bottle  in  the  hand,  as  in  the  figure,  so  that  the  upper  end  of  the  wire 


STATIC  ELECTRICITY.  31 

is  near  a  belt  giving  off  electricity.  After  a  few  seconds  touch  the 
wire  with  the  other  hand,  and  you  will  get  a  hard  shock.  The  shock 
will  be  harder  if  the  wire  is  held  near  the  belt  longer,  or  if  the  bottle 
is  larger.  This  was  first  discovered  by  a  student  in  Leyden,  and  is 
generally  known  as  the  Leyden  (pronounced  li-den)  jar.  The  more 


BELT 


FIG.  lll.-LEYDEN  JAR. 

common  form  is  a  glass  jar  coated  on  the  inside  and  outside  with  tin 
foil  up  to  within  a  few  inches  of  the  top.  The  glass  is  carefully 
kept  dry  and  sometimes  is  covered  with  shellac  or  varnish  to  keep  off 
moisture  which  would  allow  the  electricity  to  creep  along  the  sur- 
face from  one  coating  to  the  other.  Connection  with  the  inner  coat- 
ing is  made  by  means  of  a  chain  or  light  wire  attached  to  a  knob  on 
the  cork  top. 

112.     What  is  a  condenser? 

A  Leyden  jar  is  one  form  of  condenser.  The  glass  jars  are  so  ex- 
pensive, bulky  and  easily  broken,  that  another  form  is  generally 
used  when  a  large  capacity  is  desired.  The  common  form  consists 


FIG.  112.— CONDENSERS. 

of  a  number  of  sheets  of  tin  foil  separated  from  each  other  by 
sheets  of  mica  or  paper  soaked  in  oil  or  paraffine.  The  alternate 
sheets  of  tin  foil  are  connected  together,  as  indicated  in  the  figure, 
and  are  then  placed  in  a  wooden  box  into  which  melted  paraffine  is 
poured  so  that  the  whole  becomes  solid. 

113.    Must  the  surfaces  of  a  condenser  be  made  of  tin  foil? 

No.     Any  conductor  will  do.     Tin  foil  is  used  because  it  is  cheap 


32  ELECTRICAL   CATECHISM. 

and  easily  handled.      Sheet  lead  from  tea  chests,  or  tin  foil  from 
tobacco  packing  is  good  for  such  purpose. 

114.  Are  ordinary  mica  or  paraffine  condensers  suitable  for  ex* 
periments  with  electricity  from  belts  or  static  machines? 

Electricity  of  so  high  pressure  is  liable  to  puncture  the  insulation 
and  ruin  condensers  of  ordinary  construction.  Only  Leyden  jars 
or  other  specially  constructed  condensers  should  be  used  with  charges 
of  high  potential. 

115.  Are  condensers  common? 

Yes.  Any  two  conductors  which  are  near  together  and  insulated 
from  each  other,  act  as  a  condenser.  The  iron  frame  of  a  dynamo 
or  motor  and  the  wire  wound  upon  the  fields  and  armature  act  as 
the  two  surfaces  of  a  condenser,  and  one  may  sometimes  get  a  severe 
shock  by  touching  both  at  the  same  time.  A  long  insulated  wire 
buried  in  the  earth  or  in  the  water  forms  a  condenser  on  account 
of  the  surface  of  the  wire  and  the  surface  of  water  or  earth  outside 
being  so  large  and  so  close  together.  This  is  the  principal  reason 
why  it  is  impossible  to  telephone  across  the  ocean.  Much  of  the 
"cross-talk"  between  different  lines  on  the  same  pole  is  caused  by 
the  different  wires  acting  as  surfaces  of  condensers.  (See  Nos.  101, 
1 02,  103.)  A  long  lamp  cord  consisting  of  two  wires  or  cables  side 
by  side  has  so  much  capacity  that  a  strong  magneto  will  ring  through 
them,  although  the  insulation  resistance  between  them  is  millions  of 
ohms.  In  the  same  way  a  strong  magneto  will  ring  when  connected 
to  the  wire  and  to  the  sheath  of  a  lead-covered  cable,  or  between  a 
long  underground  wire  and  the  ground.  (See  Nos.  1131  to  1135.) 

116.  What  are  condensers  used  for? 

They  are  used  in  connection  with  high-speed  telegraph  systems, 
and  to  some  extent  with  telephones  and  alternating-current  motors 
and  in  connection  with  certain  attempts  to  obtain  light  without  heat 
by  electric  currents  of  very  high  frequency.  When  properly  connected 
and  when  of  suitable  capacity,  condensers  balance  the  self-induction 
of  long  lines  or  of  electro-magnets,  so  as  to  enable  signals  or  other 
rapid  variations  in  the  strength  or  direction  of  currents  to  be  sent 
faster  with  less  interference.  (This  action  is  rather  complicated, 
and  its  full  explanation  requires  an  extensive  knowledge  of  mathe- 
matics and  electricity.)  Condensers  are  also  used  in  making  accu- 
rate scientific  measurements.  Many  experiments  with  static  elec- 
tricity are  much  more  effective  when  condensers  are  used. 


STATIC  ELECTRICITY.  33 

117.  Is  there  a  simple  and  elementary  explanation  of  the  action 
of  the  condenser? 

Its  action  may  be  considered  as  a  result  of  the  mutual  attraction 
between  positive  and  negative  electricity.  The  condenser  consists 
of  two  large  conducting  surfaces  very  close  together,  such  as  two 
or  more  sheets  of  tinfoil  separated  by  mica.  When  the  two  sur- 
faces are  connected  with  opposite  terminals  of  a  source  of  electricity, 
such  as  a  battery,  a  dynamo  or  a  static  machine,  a  positive  charge 
collects  on  one  plate  and  a  negative  charge  on  the  other.  These 
charges  attract  ^one  another  so  strongly  that  a  large  quantity  will 
accumulate.  The  quantity  of  electricity  held  by  the  condenser  de- 
pends upon  the  size  of  the  surfaces,  their  nearness  together,  the  nature 
of  the  insulating  material  between  them  and  upon  the  electrical 
pressure  (sometimes  called  difference  of  potential)  between  the  ter- 
minals of  the  battery  or  other  source  of  charge.  When  the  two  sur- 
faces are  connected  by  a  conductor,  the  two  charges  run  together, 
the  positive  and  negative  charges  neutralizing  each  other,  so  that  the 
condenser  becomes  discharged. 

118.  What  is  the  formula  for  calculating  the  capacity  of  a  con- 
dens  erf 

The  capacity  equals  the  product  of  the  specific  inductive  capacity 
of  the  dielectric  multiplied  by  the  total  area  of  the  cross  section  of 
the  dielectric,  divided  by  a  constant  factor  times  the  thickness  of  the 
dielectric.  When  the  condenser  consists  of  parallel  plates  of  con- 
ductor arranged  in  piles  or  layers,  so  that  each  alternate  plate  is  con- 
nected to  the  same  terminal,  the  cross  section  of  the  dielectric  equals 
the  total  surface  (both  sides)  of  all  the  leaves  or  plates  connected 
with  one  terminal.  Taking  the  area  in  square  centimeters  and  the 
distance  between  the  plates  or  leaves  in  centimeters,  and  the  capacity 
in  microfarads  (see  Nos.  139  and  202  for  definitions  of  these  units), 
the  formula  becomes, 

~  k  A  .      ,  k  A 

C  = microfarads  = c.g.s.  units. 

1 1,310,000 1  4?rt 

Taking  the  area  in  square  inches  and  the  thickness  in  inches, 

c=      kA     . 

4,452,000  t 

1 19.  Give  an  example  of  calculating  the  capacity  of  a  condenser. 

Suppose  the  dielectric  is  paraffined  paper  having  a  specific  in- 
ductive capacity  of  1.977  and  being  0.03933  in.  thick,  the  area  of  both 


34  ELECTRICAL   CATECHISM. 

sides  of  each  set  of  leaves  being  205  sq.  ft.,  or  29,520  sq.  ins.    The 
formula  becomes, 

1.977  X  29,520 

C  =    =  0.3335  microfarads. 

4,452,000  X  0.03933 

120.  How  can  the  formula  be  changed  to  show  the  area  neces- 
sary for  a  desired  capacity? 

Using  square  centimeters  area  and  centimeters  thickness, 
11,310,000  C  t 

A  =  -  -E 

Using  square  inches  and  inches,  it  becomes 
4,452,000  C  t 

~F~ 

121.  What  is  specific  inductive  capacity? 

Specific  inductive  capacity  is  the  quality  of  a  dielectric  which  en- 
ables it  to  hold  an  electric  charge  between  two  conductors.  Air  is 
taken  as  a  standard,  its  specific  inductive  capacity  being  called  unity. 
When  two  conductors  such  as  two  parallel  plates  are  connected  to 
the  opposite  terminals  of  a  battery  or  other  source  of  potential  differ- 
ence, the  quantity  of  electricity  that  flows  into  or  out  of  the  con- 
ductors depends  upon  the  nature  of  the  substance  between  the  plates 
(see  No.  117)  ;  thus,  if  two  plates  were  separated  a  given  distance 
by  glass,  the  combination  would  constitute  a  condenser  of  about  three 
times  what  the  capacity  would  be  with  simply  air  between  the  plates, 
hence  the  specific  inductive  capacity  of  glass  is  about  three. 

122.  What  values  does  specific  inductive  capacity  have? 

The  values  vary  with  the  nature  of  the  substance  and  to  some  ex- 
tent with  the  time  during  which  the  plates  are  connected  with  the 
source  of  charge.  Values  of  common  substances  have  been  found 
as  follows: 

Vacuum  air  at  o.ooi  mm.  pressure about    0.94 

"5.  "  "         0.9985  to    0.9994 

Hydrogen  at  760  O-9997  to    0.9998 

Paraffine  wax , .   i  .68      to     2.47 

India  rubber  (pure)   2.12      to     2.34 

India  rubber  (vulcanized)  2.69      to     2.94 

Resin 2.54 

Ebonite   2.21       to     3.15 

Shellac   2.74       to     3.73 

Gutta  percha 2.46      to    4.2 

Kerosene 1.92       to    2.26 


STATIC  ELECTRICITY.  35 

Glass   3.00      to    9.9 

Mica  5.0 

Water    75.          to  84. 

123.  What  is  meant  by  a  "residual  charge?" 

After  a  condenser  has  been  discharged  and  allowed  to  stand  by 
itself  insulated  from  other  conductors,  it  is  frequently  found  to  be 
charged  again,  though  not  to  so  great  an  extent  as  at  first.  Several 
successive  discharges  may  thus  be  obtained,  the  original  charge  hav- 
ing apparently  soaked  into  the  dielectric.  For  the  same  reason,  the 
amount  of  charge  a  condenser  takes  depends  upon  the  length  of 
time  given  for  charging.  Mica  and  air  do  not  soak  up  the  charge. 

124.  Can  any  interesting  experiments  be  made  with  a  condenser, 
or  Ley  den  jar? 

One  can  get  or  give  shocks  by  them ;  make  a  person  see  stars  ;  kill 
flies  or  rats ;  pierce  holes  in  cardboard  or  even  in  glass ;  make  light- 
ning and  thunder  on  a  small  scale ;  set  fire  to  gas  or  alcohol ;  get 
pretty  luminous  effects.  Other  experiments  will  suggest  themselves 
to  the  observant  student. 

125.  How  can  a  condenser  be  used  to  give  a  shock  f 

One  method  was  suggested  in  No.  103.  A  favorite  joke  in  electric 
light  stations  is  to  charge,  or  "load,"  an  incandescent  lamp  by  hold- 
ing it  near  a  belt,  and  then  to  hand  it  to  some  unsuspecting  person ; 
or  if  the  room  is  very  dry  leave  it  in  some  convenient  place  for 
curious  persons  to  examine. 

126.  Is  it  possible  to  kill  rats  or  Hies  by  the  spark  from  a  belt? 
They  can  be  given  shocks  that  will  greatly  surprise,  and  sometimes 

stun  or  even  kill  them.  Arrange  two  strips  of  metal  side  by  side 
and  well  insulated  from  one  another,  as  suggested  in  the  figure.  Dry 
wood  will  insulate  well  enough.  Connect  one  strip  to  a  spark  col- 


FIG.  126.— ELECTROCUTION. 


lector,  such  as  a  brush  or  wire  near  the  belt,  and  connect  the  other 
end  with  the  water  pipe  or  other  good  ground.  When  any  animal 
crosses  from  one  to  the  other  in  such  a  way  as  to  touch  both  metal 
strips  at  once  he  will  regret  it.  A  similar  plan  may  be  arranged  to 


36  ELECTRICAL   CATECHISM. 

prevent  people  from  committing  nuisance.  In  a  similar  way  butch- 
ers and  candy  merchants  have  reminded  meddlesome  persons  against 
handling  their  meat  or  other  wares. 

127.  How  can  one  see  stars  by  electricity? 

A  drinking  glass  with  metal  case  such  as  used  at  soda  fountains 
makes  a  condenser  that  may  be  loaded  easily,  as  suggested  in  No. 
in,  the  liquid  and  the  metallic  holder  being  the  two  surfaces.  After 
the  cup  is  charged  (not  too  strongly  for  fear  of  injuring  the  victim), 
hand  it  to  a  friend  to  drink.  He  will  see  stars  and  will  endeavor  to 
share  the  sensation  with  the  perpetrator  of  the  joke.  One  can  also 
see  stars  by  the  help  of  an  incandescent  lamp.  Charge  a  lamp  feebly 
by  holding  the  bulb  in  the  hand  and  bringing  the  lamp  base  near  a 
belt  for  a  short  time,  being  careful  not  to  charge  it  too  strongly; 
then  touch  the  lamp  base  to  the  forehead.  One  will  seem  to  see  a 
flash  of  light,  although  the  eyes  are  tightly  closed.  The  electricity 
passing  between  the  forehead  and  hands  affects  the  optic  nerve  in 
about  the  same  way  that  light  affects  it. 

128.  How  can  luminous  effects  be  produced  by  a  condenser  or  by 
static  electricity  f 

An  incandescent  lamp — one  with  a  broken  filament  is  as  good  as 
any — will  become  luminous  if  held  in  the  hand  and  brought  near  the 
belt.  Pretty  effects  are  obtained  by  moving  the  fingers  to  different 
parts  of  the  bulb.  For  some  time  after  the  bulb  has  been  charged, 
it  may  be  made  to  glow  by  simply  rubbing  with  the  hand.  These 
effects  are  much  prettier  in  a  dark  room.  Sometimes  when  the  lamp 
is  highly  charged,  sparks  will  pass  along  the  surface  of  the  glass  or 
will  seem  to  jump  through  it,  and  will  give  one  quite  a  hard  shock. 

129.  What  is  a  Geisler  tube? 

This  is  a  glass  tube  containing  two  platinum  wire  terminals  pass- 
ing through  the  glass  and  having  the  air  exhausted  so  as  to  leave  a 
partial  vacuum.  Electric  discharges  or  sparks  pass  between  the  two 
wires  much  more  easily  in  a  vacuum  than  in  ordinary  air.  The 
passage  of  the  discharge  causes  the  air  or  other  gas  to  become  dimly 
luminous.  The  color  of  the  light  depends  upon  the  kind  of  glass 
and  upon  the  gas  inclosed.  Geisler  tubes  are  sometimes  made  in 
beautiful  designs.  An  incandescent  lamp  is  the  most  common  form 
of  Geisler  tube. 

130.  Can  Geisler  tubes  be  used  for  lighting  purposes? 

Until  recently  the  Geisler  tube  has  been  principally  an  interesting 
scientific  toy,  but  for  several  years  past  a  number  of  investigators 
have  been  trying  to  put  it  into  shape  for  lighting  purposes.  The 


STATIC  ELECTRICITY.  37 

Geisler  tube  is  the  most  efficient  source  of  artificial  light  that  is 
known ;  that  is,  a  larger  proportion  of  the  total  energy  delivered  to 
it  is  changed  into  light  than  by  any  other  known  means.  But  the 
light  from  such  tubes  is  always  very  small,  and  hence  is  not  suitable 
for  general  illumination.  Tesla,  Moore  and  others  have  done  much 
work  along  this  line,  and  it  is  one  of  the  most  promising  fields  of 
electrical  research. 

131.  How  can  lightning  and  thunder  be  produced  artificially? 
The  crackling  sound  heard  when  a  spark  passes  is  really  thunder 

on  a  small  scale,  and  the  spark  is  miniature  lightning.  The  noise 
is  caused  by  the  sudden  heating  and  cooling  of  the  air  by  the  spark. 
The  larger  the  condenser  and  the  higher  the  potential,  the  louder 
will  be  the  sound. 

132.  Hozv  can  the  spark  be  made  to  pierce  holes  in  cardboard  or 
glass? 

Paper  or  cardboard  can  be  pierced  by  connecting  the  two  ter- 
minals of  a  charged  condenser  to  two  points  directly  opposite  each 
other  and  separated  only  by  the  paper.  In  passing  between  the  two 
points,  the  sparks  make  small  holes  in  the  paper  or  cardboard.  By 
having  a  condenser  sufficiently  large  and  charged  to  a  very  high  po- 
tential, such  as  may  be  obtained  by  a  large  static  machine,  one  may 
pierce  holes  through  glass.  (See  Nos.  94  and  95.)  The  apparatus 
in  some  of  the  universities  will  pierce  glass  J  in^  thick.  By  ex- 
amining carefully  the  appearance  of  the  hole  through  paper,  card- 
board or  glass  to  discover  the  direction  in  which  the  spark  passed, 
one  sees  that  the  spark  seemed  to  go  in  both  directions  or  to  start  in 
the  middle.  The  evidence  of  extended  experiments  with  more  com- 
plicated apparatus  confirms  the  belief  that  the  spark  really  consists 
of  a  number  of  sparks  going  first  in  one  direction  and  then  in  the 
other ;  in  other  words,  the  discharge  of  a  condenser  is  often  oscilla- 
tory, the  spark  passing  back  and  forth  like  the  swinging  of  a 
pendulum.  Careful  study  of  experiments  along  this  line  has  led 
scientists  to  believe  that  lightning  discharges  are  generally  oscilla- 
tory, and  the  most  efficient  lightning  arresters  are  made  upon  that 
idea.  Further  reasoning  from  this  point  strengthens  the  theory 
and  belief  that  electricity  itself  is  a  vibratory  motion,  and  almost  if 
not  quite  identical  with  light. 

133.  How  can  alcohol  be  set  afire  by  a  spark? 

Put  the  alcohol  in  a  dish  in  which  a  wire  or  other  conductor  is 
placed,  so  that  the  end  is  at  or  just  below  the  surface  of  the  alcohol. 
Connect  the  further  end  of  the  wire  to  one  terminal  of  the  charged 


38  ELECTRICAL  CATECHISM. 

condenser  and  bring  the  other  terminal  near,  so  that  a  spark  will 
pass  to  the  point  in  the  alcohol.  The  spark  at  the  surface  of  the  alco- 
hol and  air  will  set  it  afire.  Cigar  lighters  are  sometimes  made 
upon  this  principle,  the  spark  being  produced  by  a  frictional  ma- 
chine or  by  an  induction  coil. 

134.  How  can  gas  be  lighted  by  a  condenser? 

This  is  practically  the  same  thing  as  described  in  Nos.  43  and  54. 
Connect  one  terminal  of  the  condenser  to  the  gas-pipe  by  means  of  a 
wire,  chain  or  wet  string ;  turn  on  the  gas  and  bring  the  other  ter- 
minal of  the  condenser  near  the  gas  jet,  so  that  the  spark  will  pass 
through  the  stream  of  gas.  If  you  do  not  mind  a  shock,  hold  one 
terminal  of  the  condenser  in  one  hand,  turn  on  the  gas  and  bring  the 
other  condenser  terminal  near  the  gas  jet  while  still  holding  the  gas 
cock.  The  discharge  will  then  pass  through  your  arms. 

135.  Can  static  electricity  be  measured? 

Both  the  quantity  and  the  pressure  of  static  electricity  can  be 
measured  by  suitable  instruments. 

136.  What  units  are  used  in  measuring  static  electricity? 

There  are  two  systems  of  units,  the  electrostatic  and  the  electro- 
magnetic. The  electrostatic  units  are  not  often  used,  except  in 
purely  theoretical  and  mathematical  work.  The  electromagnetic 
units  used  in  connection  with  static  electricity  are  the  coulomb,  the 
volt  and  the  farad  or  microfarad.  (See  also  No.  224.) 

137.  What  is  a  coulomb? 

The  coulomb  is  the  electromagnetic  unit  of  quantity  of  electricity. 
It  is  3,000,000,000  times  the  electrostatic  unit  of  quantity,  which  is 
the  quantity  which  would  repel  an  equal  quantity  at  a  distance  of  one 
cm  with  a  force  of  i  dyne.  The  coulomb  is  the  quantity  of  electricity 
represented  by  a  current  of  I  ampere  during  one  second  of  time. 
It  is  the  quantity  of  electricity  in  a  condenser  of  one  farad 
capacity  when  charged  at  a  pressure  of  I  volt.  The  unit  is 
named  after  Charles  A.  Coulomb,  a  celebrated  physicist,  who  lived 
from  1736  to  1806.  (See  also  No.  229.) 

138.  What  is  a  volt? 

The  volt  is  the  unit  of  pressure  or  electromotive  force.  It  is  the 
pressure  required  to  force  one  coulomb  of  electricity  into  a  condenser 
of  one  farad  capacity.  A  "Clark  standard  cell"  has  an  electromo- 
tive force  of  1.434  volts.  The  pressure  of  a  sal  ammoniac  or  Le- 
clanche  cell  of  battery  is  about  1.4  volts ;  that  of  a  gravity  or  "Crow- 
foot" cell  is  about  i.i  volts.  The  volt  is  the  electromotive  force 


STATIC  ELECTRICITY.  39 

set  up  in  a  conductor  that  "cuts"  or  crosses  magnetic  lines  of  force 
at  the  rate  of  100,000,000  per  second,  or  a  coil  in  which  the  number 
of  magnetic  lines  inclosed  changes  at  the  same  rate.  The  volt  is 
named  after  Alessandro  Volta,  an  Italian  physicist,  who  lived  from 
1745  to  1827.  (See  also  Nos.  228  and  235.) 

139.  What  is  a  microfarad? 

It  is  the  practical  unit  of  capacity,  being  one  millionth  part  of 
the  farad.  The  standards  actually  used  consist  of  sheets  of  tin  foil 
separated  by  thin  mica,  the  layers  or  sheets  being  connected  alter- 
nately, as  indicated  in  Fig.  112.  The  farad  is  named  after  Michael 
Faraday,  a  celebrated  British  physicist  of  the  early  part  of  the  nine- 
teenth century,  1791  to  1867.  (See  also  No.  230.) 

140.  How  can  a  quantity  of  static  electricity  be  measured? 

It  can  not  easily  be  measured  directly.  The  most  common  method 
is  to  measure  the  current  it  will  cause  for  a  short  time.  This  is  done 
by  the  use  of  a  ballistic  galvanometer.  The  quantity  can  be  calcu- 
lated from  the  capacity  and  pressure,  as  the  number  of  coulombs 
equals  the  product  of  the  number  of  farads  by  the  number  of  volts. 

141.  What  is  a  ballistic  galvanometer? 

The  ballistic  galvanometer  has  a  comparatively  heavy  moving  part 
that  swings  so  slowly  that  the  transient  current  from  the  discharge 
of  a  condenser  or  similar  source  can  practically  all  pass  through  the 
coil  before  the  moving  part  has  moved  sensibly  away  from  the  zero 
point.  (See  No.  832  for  further  description  of  galvanometer.)  The 
throw  of  the  needle,  that  is,  the  amount  it  is  deflected  at  the  first 
swing,  is  proportional  to  the  quantity  of  electricity  that  passes 
through  the  coil. 

142.  How  can  the  pressure  of  static  electricity  be  measured? 
The  pressure  or  potential  can  be  accurately  measured  by  various 

forms  of  electrometers.  Electrometers  are  based  upon  the  simple 
principle  that  there  is  an  attraction  between  bodies  at  different  po- 
tentials, or,  as  sometimes  stated  (see  No.  25),  that  bodies  similarly 
charged  repel  each  other,  while  bodies  oppositely  charged  attract. 
The  commercial  forms  are  known  as  "electrostatic  voltmeters." 

143.  Describe  the  electrostatic  voltmeter. 

The  simple  form  is  illustrated  in  the  figure.  It  consists  of  two 
butterfly  shaped  sheets  of  brass  parallel  to  each  other  and  metallic- 
ally connected  together  and  to  one  binding  post,  but  carefully  in- 
sulated from  the  rest  of  the  instrument.  Midway  between  them  is 
the  "needle,"  a  light  and  thin  strip  of  aluminum  pivoted  on  knife 


40  ELECTRICAL   CATECHISM. 

edges  and  carrying  a  light  pointer  at  the  upper  end.  The  lower  half 
is  slightly  heavier  than  the  upper  half,  so  that  the  pointer  stands  at 
zero.  The  needle  is  carefully  insulated  from  the  stationary  plates 
and  is  connected  with  the  other  binding  post.  When  the  two  bind- 
ing posts  are  connected  with  the  terminals  of  a  dynamo  or  any  other 
points  having  a  considerable  difference  of  potential,  the  plates  and 


FIG.  143.-ELECTROSTATIC  VOLTMETER. 

the  needle  are  charged  to  corresponding  differences  of  potential  and 
attract  one  another.  The  needle,  therefore,  moves  so  as  to  get  closer 
to  the  plates,  or,  what  is  the  same  thing,  swings  on  the  knife  edges 
so  that  a  larger  part  of  the  needles  is  between  the  stationary  plates. 
Since  the  lower  half  of  the  needle  is  heavier  than  the  upper,  any  mo- 
tion of  the  needle  means  lifting  a  weight,  so  that  the  electrostatic  at- 
traction is  balanced  by  gravity.  The  amount  of  the  motion  of  the 
needle,  therefore,  measures  the  difference  of  potential  between  the 
needle  and  the  stationary  plates,  that  is,  between  the  terminals  of 
the  instrument.  The  form  of  voltmeter  shown  in  the  figure  is  suit- 
able for  differences  of  potential  between  200  and  20,000  volts.  An- 
other style  is  made  for  potentials  up  to  100,000  volts.  (See  also 
Nos.  801,  802  and  803.) 

144.  Can  electrostatic  voltmeters  be  made  to  measure  less  than 
200  volts? 

The  multicellular  form  is  used  for  pressures  between  40  and  1600 
volts.  The  figure  shows  one  style  of  Thomson  or  Kelvin  multi- 
cellular  voltmeter.  The  moving  plates  are  fastened  to  a  stiff  ver- 
tical wire  which  is  suspended  by  a  fine  wire  that  becomes  twisted 


STATIC  ELECTRICITY.  41 

slightly  when  the  moving  part  is  drawn  in  between  the  stationary 
plates.     The  torsion  of  the  suspension  wire  is  the  controlling  force 


FIG.  144.— MULTI CELLULAR  VOLTMETER. 

in  this  instrument.     *A  piece  of  metal  hangs  in  a  dish  of  glycerine 
or  oil  below  to  make  the  instrument  more  dead-beat. 

145.  Is  not  the  electrometer  sometimes  made  for  measuring  small 
differences  of  potential? 

The  "quadrant  electrometer,"  designed  by  Lord  Kelvin,  is  some- 
times made  so  sensitive  as  to  measure  pressures  as  low  as  i/iooo 
volt.  It  is  only  useful  for  laboratory  work  in  the  hands  of  skilful 
observers. 

146.  What  are  the  advantages  of  electrostatic  instruments? 

They  are  suitable  for  measuring  either  direct  or  alternating  pres- 
sures (except  that  the  quadrant  electrometer  is  not  so  sensitive  for 
very  small  alternating  pressures),  since  the  attraction  depends  upon 
the  difference  of  potential  between  the  plates  and  is  independent  of 
their  relative  polarity.  Another  advantage  is  that  it  takes  no  per- 
ceptible current  or  energy,  and  so  does  not  become  heated  in  use. 
It  is  especially  valuable  in  cases  where  there  is  no  current  or  where 
such  is  not  desirable. 

147.  Can  an  electrometer  be  used  to  measure  different  kinds  of 
electricity? 

The  electrometer  will  measure  differences  of  potential,  no  matter 
from  what  source.  It  shows  that  the  electricity  obtained  by  friction 
is  the  same  as  that  obtained  from  chemical  action  in  batteries  or 
by  induction  in  dynamo  machines.  The  electricity  from  friction  is 


42  ELECTRICAL   CATECHISM. 

small  in  quantity,  but  of  high  pressure  or  electromotive  force ;  while 
that  from  batteries  is  of  comparatively  low  pressure,  but  may  be  of 
considerable  quantity.  By  connecting  several  thousand  battery  cells 
in  series,  one  may  obtain  precisely  the  same  effects  as  from  belts  or 
friction.  Also  by  proper  arrangements,  one  can  obtain  from  a 
sou.rce  of  static  .electricity,  such  as  a  belt,  the  same  effects  as  are  pro- 
duced by  a  small  current  from  a  battery.  (See  Nos.  4  and  158.) 

T48.     /.$•  the  air  itself  electrified? 

It  seems  so.  Probably  the  electricity  in  clear  air  is  carried  by 
the  invisible  particles  of  moisture  always  present.  The  atmospheric 
electricity  undoubtedly  has  much  to  do  with  the  growth  of  plants 
and  animals.  The  sharp  spines  on  the  edges  of  leaves  and  the  fine 
hair  on  most  plants  seem  to  be  partly  for  the  purpose  of  collecting 
electricity  from  the  air.  It  is  found  that  under  shade  trees  the  air 
is  less  electrified  than  away  from  them,  and  many  eminent  scientists 
believe  that  is  why  grass  does  not  grow  so  well  under  trees  as  in  open 
fields.  They  think  the  shade  is  not  the  only  reason,  and  perhaps  not 
the  principal  one.  Probably  the  reason  why  nothing  grows  well 
under  pine  or  cedar  trees  is  that  the  many  sharp  points  of  the  pine 
needles  deprive  the  surrounding  air  of  its  electricity.  It  is  found 
that  the  evaporation  of  water  from  animals  and  plants  is  much  faster 
in  electrified  air. 

149.     What  causes  the  lightning? 

The  clouds  are  more  or  less  charged  with  electricity.  When  the 
difference  of  potential  between  two  clouds  or  between  a  cloud  and 
the  earth  becomes  very  great,  the  air  can  not  stand  the  strain  and 
"breaks  down,"  allowing  a  discharge  to  occur  between  the  two. 
Much  study  has  been  given  to  this  subject  and  much  remains  to  be 
learned.  Several  theories  have  been  proposed  to  explain  the  elec- 
trification of  the  air.  When  water  evaporates,  the  particles  of  moist- 
ure are  found  to  be  electrified.  When  these  particles  collect  and 
form  clouds  their  charges  unite,  each  drop  of  water  containing  the 
charges  of  the  particles  composing  it.  As  the  drops  get  larger  and 
larger,  their  potential  also  rises  and  the  whole  cloud  is  electrified 
more  and  more  strongly.  It  is  often  noticed  during  a  thunder 
shower  that  the  raindrops  are  larger  just  after  a  lightning  flash 
overhead.  The  peculiar  yellowish  color  in  the  clouds  during  a 
thunder  storm  is  probably  caused  by  small  discharges  between  drops 
or  between  small  clouds  in  the  larger  cloud  mass. 


STATIC  ELECTRICITY.  43 

150.  What  causes  the  "heat  lightning"  often  seen  summer  even- 
ings when  there  is  no  thunder  storm  near? 

This  is  the  reflection  from  lightning  in  distant  storms.  In  one  in- 
vestigation it  was  found  the  "heat  lightning"  was  reflected  from  a 
storm  over  100  miles  away. 

151.  Does  the  aurora  have  anything  to  do  with  electricity? 
Auroral  displays,   or  "Northern  Lights,"  are  generally  connected 

with  "magnetic  storms,"  which  affect  compass  needles  and  long  tele- 
phone and  telegraph  wires.  The  exact  cause  is  not  certain,  but  it 
is  believed  to  be  "electrical  discharges  in  the  upper  air,  in  conse- 
quence of  the  differing  electrical  conditions  between  the  cold  air  of 
the  polar  regions  and  the  warmer  streams  of  air  in  vapor  raised  from 
the  level  of  the  ocean  in  tropical  regions  by  the  heat  of  the  sun." 
The  auroral  displays  seem  to  be  intimately  associated  with  sunspots. 

152.  What  connection  is  there  between  the  aurora  and  magnetic 
storms? 

The  two  generally  occur  together.  The  magnetic  disturbances 
are  probably  caused  by  currents  in  the  earth,  set  up  by  induction  from 
the  electrical  discharges  of  the  aurora.  Such  currents  sometimes 
greatly  interfere  with  the  operation  of  long  telephone  and  telegraph 
lines. 


:53-  What  causes  the  humming  of  telegraph  and  telephone 
it  ires? 

This  is  principally  due  to  mechanical  vibration  caused  by  the  wind, 
these  vibrations  often  travelling  along  the  wires  for  miles. 

154.  Has  atmospheric  electricity  ever  been  used,  or  is  it  likely 
to  be? 

The  quiet  effects  of  the  electricity  are  doubtless  of  great  value  in 
agriculture.  The  remarkably  rapid  growth  of  vegetation  during 
the  short  summers  in  Finland  and  other  northern  countries  is  be- 
lieved to  be  due  largely  to  the  great  amount  of  atmospheric  elec- 
tricity present.  (See  also  Nos.  no  and  148).  The  lightning  appar- 
ently causes  the  formation  of  more  or  less  nitric  acid  and  ammonia 
found  in  the  air  after  thunder  showers.  Many  interesting  experiments 
have  been  made.  Probably  the  largest  commercial  enterprise  yet 
developed  from  atmospheric  electricity  is  the  manufacture  of  light- 
ning rods  and  lightning  arresters. 

155.  Are  lightning  rods  of  any  value  except  to  the  agent? 
When  properly  put  up  they  are  undoubtedly  a  source  of  much  pro-' 

tection.     The  conductor  should  be  made  of  galvanized  iron  rather 


44  ELECTRICAL   CATECHISM. 

than  of  copper :  flat  strip  is  some  better  than  round  or  fancy  star 
shape.  Sharp  bends  and  corners  in  the  conductor  should  be  avoided. 
The  conductor  should  be  carried  to  all  high  points  of  a  building, 
should  be  insulated  from  the  building,  and  should  be  well  connected 
with  good,  deep,  wet  earth,  independent  of  gas  or  water  pipes.  All 
metal  work  on  the  outside  of  the  building,  such  as  water  pipes,  iron 
cresting,  etc.,  should  be  connected  together  and  to  the  earth,  but  not 
to  the  lightning  conductor.  "The  cheapest  way  of  protecting  an  or- 
dinary house  is  to  run  common  galvanized  iron  telegraph  wire  up 
all  the  corners,  along  all  the  ridges  and  eaves  and  over  all  the  chim- 
neys ;  taking  them  down  to  the  earth  in  several  places  to  damp 
ground,  and  at  each  place  burying  a  load  of  broken  coke  or  charcoal 
to  hold  moisture."  Lightning  rods  should  not  be  left  on  buildings 
after  the  connection  with  the  earth  is  broken  or  removed.  It  is 
found  that  the  network  of  wires  used  in  cities  for  telephone,  tele- 
graph and  other  purposes  is  a  great  protection.  Cases  of  buildings 
and  trees  being  struck  by  lightning  are  much  less  frequent  in  such 
cities  than  in  the  country. 

156.  What  is  the  principle  of  the  lightning  arresters  used  for  pro- 
tecting electrical  apparatus? 

Many  of  these  depend  upon  the  fact  that  lightning  discharges  will 
jump  across  air  spaces  that  are  good  insulators  for  the  regular  work- 
ing current,  while  they  find  difficulty  in  passing  through  circuits 

TO   INSTRUMENT 

FUSE 

TO  LINE 


CARBON  BLOCK 
MICA 
CARBON  BLOCK 


TO  GROUND 
FIG.  156.— LIGHTNING  ARRESTER. 

containing  electromagnets.  The  figure  shows  an  excellent  protector 
for  telephone  instruments,  the  lightning  arrester  consisting  of  two 
blocks  of  carbon  separated  a  small  distance  by  a  thin  sheet  of  insu- 
lating mica,  which  is  perforated  with  one  or  more  holes ;  a  high  po- 
tential charge  on  the  line  will  jump  through  the  hole  in  the  mica 


STATIC  ELECTRICITY.  45 

from  the  carbon  on  the  line  side  to  the  lower  carbon,  which  is  con- 
nected with  the  ground;  the  fuses  protect  the  instruments  against 
foreign  currents  which  might  damage,  although  not  of  sufficiently 
high  potential  to  jump  to  earth;  sometimes  the  connections  are  re- 
versed so  that  the  fuse  is  between  the  line  and  the  earth.  The  com- 
plete theory  of  lightning  arresters,  is  complicated,  and  cannot  be  un- 
derstood without  a  broad  knowledge  of  electricity.  The  aim  of  the 
arresters  is  to  provide  a  path  for  lightning  to  get  to  the  earth  with- 
out going  through  the  instruments  or  other  apparatus.  Arresters 
for  electric  light  or  power  circuit  also  require  some  arrangement  to 
prevent  the  regular  working  current  from  following  the  lightning. 

157.  Does  lightning  often  strike  electric  light 'or  telephone  lines? 
Lines  are  very  rarely  struck  with  actual  lightning  from  the  clouds 

in  the  way  that  trees  are  struck.  The  lines  become  charged  to  a 
high  potential  by  induction  from  lightning  flashes  or  from  the  pass- 
ing of  clouds  that  are  highly  charged.  The  induction  from  the  pres- 
ence of  clouds  is  similar  to  that  described  in  No.  99. 

158.  What  is  the  difference  between  static  electricity  and  current? 
One  is  electricity  at  rest,  the  other  is  electricity  in  motion.    When 

static  electricity  is  properly  handled,  it  will  produce  most  of  the  re- 
sults obtained  from  currents.  If  the  two  surfaces  of  a  charged  con- 
denser are  connected  by  wire  or  other  conductor,  the  charge 
on  one  surface  passes  around  to  the  other  and  unites  with 
or  neutralize  it.  While  this  discharge  is  taking  place,  an 
electric  current  is  moving  through  the  wire  from  the  positive  side 
of  the  condenser  to  the  negative  side.  This  current  lasts  only  a 
short  time,  usually  only  a  small  part  of  a  second,  but  while  it  lasts 
it  heats  the  wire  and  gives  magnetic  effects  much  the  same  as  an  or- 
dinary current.  Likewise  the  effects  of  static  electricity  may  be 
produced  by  current  electricity  if  the  voltage  is  very  high,  several 
thousand  volts.  Even  with  low  voltage,  some  static  effects  can 
be  produced,  as  illustrated  in  answers  94  to  98. 


CHAPTER  II. 


UNITS. 

200.  What  is  the  "metric"  or  "C.  G.  S."  system  of  units? 

This  is  a  system  worked  out  in  Paris  about  a  hundred  years  ago 
and  gradually  adopted  by  scientists  in  all  parts  of  the  world.  It  is 
legalized,  but  not  made  compulsory,  by  Congress.  The  name  "C 
G.  S."  is  an  abbreviation  of  "centimeter-gram-second,"  indicating  the 
principal  units  of  the  system. 

201.  Why  do  so  many  electrical  engineers  use  the  metric  system? 
Because  its  units  are  regularly  related  on  the  decimal  system,  so 

that  it  is  easy  to  pass  from  one  unit  to  another  without  so  many  and 
tedious  calculations,  as  are  necessary  in  the  English  system.  The 
electrical  units  are  derived  from  the  "C.  G.  S."  units  and  for  the  sake 
of  uniformity,  as  well  as  convenience,  electrical  engineers  use  the 
same  system  as  other  scientists.  On  account  of  the  common  use  of 
the  English  pound  and  inch  in  the  shops,  it  is  frequently  necessary 
to  use  the  English  units  in  giving  dimensions,  so  that  both  systems 
are  used  together. 

202.  What  is  the  centimeter? 

The  centimeter  is  the  unit  of  length  and  was  intended  to  be  the  one 
thousand-millionth  part  of  the  distance  from  the  equator  to  the  pole 
of  the  earth.  Practically,  it  is  the  one  hundredth  part  of  the  distance 
between  two  parallel  lines  ruled  on  a  bar  of  platinum  which  is  care- 
fully preserved  in  the  Archives  of  Paris.  The  word  "meter"  is  de- 
rived from  Greek,  Latin  and  French  words  meaning  "measure." 

203.  What  are  the  other  units  of  length  used  in  the  metric  sys- 
tem? 

The  millimeter  is  one-tenth  part  of  the  centimeter.  The  meter 
is  100  cm.  The  kilometer  is  1000  meters. 

204.  What  is  the  English  equivalent  of  the  centimeter? 

The  centimeter  equals  0.393704  inch,  or  I  in.  equals  about  2.5  cm. 
A  meter  is  about  39.4  ins.  A  kilometer  is  about  f  mile.  A  square 
centimeter  is  0.155  sq.  ins.,  and  a  square  inch  equals  6.452  sq.  cm. 


UNITS. 


47 


205.  What  is  the  gram? 

The  gram,  or  gramme  (from  the  Greek  and  Latin  gramma,  "a 
small  weight"),  is  the  unit  of  mass  and  also  of  weight,  that  is,  of 
the  force  with  which  the  mass  is  attracted  by  the  earth.  It  is  the  mass 
or  weight  of  a  cubic  centimeter  of  water  at  4  deg.  C,  or  39.2  deg. 
F.,  the  temperature  at  which  its  density  is  greatest. 

206.  What  is  the  English  equivalent  of  the  gram? 

The  gram  equals    15432   gr.,   or  0.03527  oz.,   or  0.002204  lb., 


AMPERE 


avoirdupois.  The  kilogram  (1000  grams)  equals  about  2.2  Ibs.  One 
ounce  avoirdupois  equals  28.35  grams.  One  avoirdupois  pound 
equals  0.4536  kilogram  of  453.6  grams. 

207.     What  is  the  second? 

The  second  is  the  unit  of  time,  and  is  the  same  in  the  metric  and 
English  systems.  It  is  the  time  of  a  single  swing  of  a  pendulum 
0.99355  meter  long  at  sea  level,  45  deg.  latitude,  and  is  1-86400  of 
the  mean  time  from  noon  to  noon,  that  is,  of  a  mean  solar  day. 


ELECTRICAL   CATECHISM. 


208.     What  is  the  C.  G.  S.  unit  of  force? 

The  dyne  (derived  from  a  Greek  word  meaning  "force")  is  the 
metric  or  C.  G.  S.  unit  of  force.  It  is  the  force  which,  when  acting 
for  one  second  on  a  mass  of  one  gram,  gives  it  a  velocity  of  I  cm. 
per  second.  The  earth  attracts  every  particle  at  its  surface  with  a 
force  of  978  to  983  dynes  for  each  gram  of  mass. 


209.     What  is  the  C.  G.  S.  unit  of  work? 
The  unit  of  work  and  of  energy  is  the  erg. 


It  is  the  work  done  tr 


VOLTA 


a  force  of  one  dyne  acting  through  a  distance  of  I  cm.,  or,  it  is  the 
work  done  in  pushing  a  body  through  a  distance  of  I  cm.  against  a 
force  of  i  dyne.  The  work  of  lifting  I  gram  I  cm.  is  about  980  ergs. 
(Erg  is  derived  from  the  Greek  word  ergon,  meaning  "work.") 

210.     What  is  the  practical  unit  of  work? 

There  are  two  units  in  each  system,  the  joule  and  the  kilogram- 
meter  in  the  metric  system,  and  the  foot-pound  and  horse-power-hour 
in  the  English.  The  joule  equals  10,000,000  ergs.  The  kilogram- 
meter  equals  about  98,050,000  ergs,  depending  upon  the  strength  of 


UNITS. 


49 


gravity.  The  English  unit  is  the  foot-pound,  which  is  the  work 
done  in  lifting  I  Ib.  i  ft.  against  the  force  of  gravity.  The  horse- 
power-hour is  a  larger  unit,  equal  to  the  work  of  lifting  1,980,000 
ft.-lbs.  The  foot-pound  unit  equals  about  13,560,000  ergs,  or  0.138 
kilogram-meters.  The  kilogram-meter  equals  7.233  ft.-lbs. 

211.     What  is  the  difference  between  work  and  energy ? 

Energy  may  be  considered  as  a  cause  of  which  work  is  the  resu1t 
Both  are  measured  in  the  same  units,  the  erg,  joule  or  the  kilogram- 


OHM 


meter  in  the  metric  system,  or  the  foot-pound  in  the  English  system. 

212.  What  is  the  difference  between  work  and  power? 

Power  is  the  rate  of  doing  work.  To  raise  55  Ibs.  10  ft.  would  re- 
quire  550  ft.-lbs.  of  energy.  To  do  this  in  one  second  would  require 
550  ft-lb.-seconds,  or  I  hp.  To  raise  the  same  weight  an  equal  dis- 
tance in  one  hour,  or  3600  seconds,  would  require  only  1-3600  hp. 
The  same  work  is  done  in  the  two  cases,  but  the  power  is  different, 
because  more  time  is  taken  in  the  second  case. 

213.  What  is  the  metric  unit  of  power? 

The  unit  is  the  watt,  which  equals  one  joule  per  second.     A  larger 


50 


ELECTRICAL   CATECHISM. 


unit  is  often  used,  the  kilowatt,  which  equals  1000  watts.  The  number 
of  watts  equals  the  product  of  amperes  by  volts,  or  the  product  of 
resistance  by  the  current  squared.  The  joule  was  named  after  James 
Prescott  Joule  (1818  to  1889),  an  English  physicist  who  did  much 
to  verify  the  law  of  conservation  of  energy.  The  watt  was  named 
after  James  Watt  (1736  to  1819),  who  greatly  improved  the  steam 
engine,  being  practically  the  inventor  of  the  modern  engine.  (See 
Nos.  231,  232.) 


WATT 


214.     What  is  the  English  unit  of  power? 

The  English  unit  is  the  foot-pound-per-second,  or  foot-pound-per- 
minute,  and  equals  the  power  required  to  raise  a  weight  of  I  avoir- 
dupois Ib.  i  ft.  per  second,  or  per  minute,  against  the  attraction  of 
gravity.  The  watt  equals  0.737  ft.-lbs.  per  second,  or  44.239  ft.-lbs. 
per  minute.  The  horse-power  is  frequently  used  as  a  unit,  being 
the  power  required  to  do  the  work  at  the  rate  of  550  ft.-lbs.  per 
second,  or  33,000  ft.-lbs.  per  minute. 


UNITS. 


51 


215.  How  much  power  is  required  to  raise  a  weight  of  20,000 
pounds  a  distance  of  20  feetf 

It  depends  upon  the  time  in  which  it  is  to  be  done.  A  small  boy 
could  raise  it  with  a  differential  pulley  of  sufficient  multiplying 
power ;  a  team  of  horses  could  raise  it  in  a  much  shorter  time.  The 
power  increases  as  the  time  allowed  decreases. 

216.     What  is  the  relation  between  watts  and  horse-power? 

The  horse-power  equals  practically  746  watts,  or  more  exactly, 
745.941  watts. 


COULOMB 


217.  Is  the  horse-power  equal  to  the  power  of  one  horse? 

It  was  originally  intended  to  be  the  power  that  could  be  con- 
tinuously exerted  by  a  good  strong  horse,  just  as  the  foot  was  taken 
as  the  average  length  of  men's  feet.  As  engineering  and  commerce 
became  of  more  importance,  it  became  necessary  to  have  more  definite 
standards,  and  so  the  foot  and  the  pound  came  to  be  copies  of  stand- 
ards kept  by  various  governments. 

218.  What  is  a  kilowatt-hour? 

This  is  the  unit  commonly  used  in  measuring  electrical  energy. 


ELECTRICAL   CATECHISM. 


It  is  the  energy  represented  by  I  kw.  operating-  for  one  hour.  For 
example,  50  kw-hours  might  mean  50  kw.  for  one  hour,  or  I  kw.  for 
fifty  hours,  or  200  kw.  for  a  quarter-hour.  (See  No.  210.) 

219.     IV hat  is  the  difference  between  a  kilowatt-hour  and  a  horse- 
power-hour? 

The  same  ratio  exists  as  between  the  kilowatt  and  the  horse-power. 
The  horse-power  represents  746  watts,  therefore,  i  hp-hour  repre 
sents  746  watt-hours,  or  0.746  kw.hour. 


-  ;-  ••     '•     -  . 


JOULE 


220.  What  is  the  relation  between  heat  and  work? 

The  English  unit  of  heat  is  that  quantity  required  to  raise  the 
temperature  of  one  pound  of  water  one  degree  Fahrenheit  at  the 
temperature  of  greatest  density,  39.1  deg.  Fahr.  This  is  called  the 
"  British  Thermal  Unit,"  and  is  usually  referred  to  as  "  B.  T.  U." 
or  as  "  B.  t.  u."  Prof.  H.  A.  Rowland  experimentally  determined 
its  value,  which  is  equal  to  778.104  foot  pounds,  often  taken  as  778. 

221.  What  is  the  metric  unit  of  heat? 

The  calorie  is  the  work  required  to  raise  the  temperature  of  one 
gram  of  water  one  degree  centigrade  at  or  near  the  temperature  of 


UNITS. 


53 


4  deg.  cent.  It  equals  41,861,700  ergs,  or  4.2  joules  or  volt-ampere- 
seconds  or  watt-seconds.  It  is  sometimes  called  the  gram-calorie 
or  lesser  calorie,  to  distinguish  it  from  the  greater  calorie  which  is 
looo  times  larger. 

222.  What  is  the  difference  between  Fahrenheit  and  Centigrade 
thermometers? 

Fahrenheit  (a  German  physicist  living  from  1686  to  1736)  took 
what  he  supposed  was  the  lowest  possible  degree  of  cold  and  called  it 


FARADAY 


zero.     He  then  divided  the  range  between  the  freezing  and  boiling 
points  of  water  at  ordinary  atmospheric  pressure  into  180  equal  parts 


I    I    II.  I     I     I     ' 

S    8    §2  S      o    S     § 


§     S 


I       i 
§     S     I 


and  continued  the  scale  to  the  zero  point,  which  made  the  freezing 
point  come  at  32  degs.      The  Centigrade  scale  was  worked  out  in 


54 


ELECTRICAL   CATECHISM. 


1741  by  Prof.  Anders  Celsius,  of  Upsala,  Sweden,  who  divided  the 
range  between  freezing  and  boiling  into  one  hundred  equal  parts. 
The  Centigrade  thermometer  is  commonly  used  in  scientific  work, 
while  the  Fahrenheit  scale  is  more  often  used  by  engineers  who  are 
accustomed  to  the  English  scales  and  units. 

223.     How  can  degrees  be  changed  from  one  scale  to  the  other? 

To  reduce  degrees  Fahrenheit  to  degrees  Centigrade,  subtract  32 
and  multiply  by  5/9.  To  reduce  degrees  Centigrade  to  degrees 
Fahrenheit,  multiply  by  9/5  and  add  32  degs. 


HENRY 


224.     What  systems  of  electrical  units  are  in  use? 

The  electrostatic  and  the  electromagnetic  units.  The  former  system 
is  derived  from  considering  as  a  unit  the  amount  of  static  electricity 
that  will  repel  or  attract  an  equal  amount  I  cm.  away  with  a  force  of 
I  dyne.  The  electromagnetic  units  are  derived  by  considering  as  a 
unit  magnetic  pole  one  that  will  attract  or  repel  a  similar  pole  at  a 
distance  of  I  cm.  with  a  force  of  i  dyne.  The  "absolute"  or  "C.  G. 
S."  electromagnetic  unit  of  quantity  of  electricity  is  30,000,000,000 
times  the  electrostatic  unit  of  quantity,  this  factor  being  closely  equal 


UNITS. 


55 


to  the  velocity  of  light  in  centimeters  per  second.  Ratios  between 
other  corresponding  units  are  derived  from  this.  For  most  purposes 
the  electromagnetic  units  are  used,  or,  more  commonly,  the  practical 
units  derived  from  them.  (See  also  Nos.  136  and  246.) 

225.     What  is  meant  by  the  international  electrical  units? 

These  are  units  recommended  by  the  International  Congress  of 
Electricians  held  in  Chicago  during  the  World's  Fair  of  1893,  and 
later  adopted  by  various  governments,  the  United  States  Congress 


MAXWELL 


approving  and  adopting  them  in  June,  1894.      These  units  include 
the  ohm,  ampere,  volt,  coulomb,  farad,  joule,  watt  and  henry. 

226.     What  is  the  international  ohm? 

"The  unit  of  resistance  shall  be  what  is  known  as  the  international 
ohm,  which  is  substantially  equal  to  one  thousand  million  units  of 
resistance  of  the  centimeter-gram-second  system  of  electromagnetic 
units,  and  is  represented  by  the  resistance  offered  to  an  unvarying 
electric  current  by  a  column  of  mercury  at  the  temperature  of  melting 
ice  fourteen  and  four  thousand  five  hundred  and  twenty-one  ten- 


56 


ELECTRICAL   CATECHISM. 


thousandths  grams  in  mass,  of  a  constant  cross  sectional  area,  and 
of  the  length  of  one  hundred  and  six  and  three-tenths  centimeters." 
(See  also  Nos.  234,  237,  315,  318.)  This  unit  is  named  after  George 
Simon  Ohm,  a  German  mathematician,  who  lived  from  1789  to  1854. 

227,     What  is  the  international  ampere? 

"The  unit  of  current  shall  be  what  is  known  as  the  international 
ampere,  which  is  one-tenth  of  the  unit  of  current  of  the  centimeter- 
gram-second  system  of  electromagnetic  units,  and  is  the  practical 


GAUSS 


equivalent  of  the  unvarying  current,  which,  when  passed  through  a 
solution  of  nitrate  of  silver  in  water,  in  accordance  with  standard 
specifications,  deposits  silver  at  the  rate  of  one  thousand  one  hundred 
and  eighteen  millionths  of  a  gram  per  second."  (See  also  Nos.  236, 
241,  317.)  This  unit  is  named  after  Andre  Marie  Ampere,  a  French 
physicist  who  lived  from  1775  to  1836. 

228.     What  is  the  international  volt? 

"The  unit  of  electromotive  force  shall  be  what  is  known  as  the 
international  volt,  which  is  the  electromotive  force  that,  steadily  ap- 


UNITS.  57 

plied  to  a  conductor  whose  resistance  is  one  international  ohm,  will 
produce  a  current  of  an  international  ampere,  and  is  practically  equiv- 
alent to  one  thousand  fourteen  hundred  and  thirty-fourths,  ( ), 

V 14347 

of  the  electromotive  force  between  the  poles  or  electrodes  of  the  vol- 
taic cell  known  as  Clark's  cell,  at  a  temperature  of  15  degs.  C,  and 
prepared  in  the  manner  described  in  the  standard  specifications." 
(See  also  Nos.  138,  235.)  The  volt  is  named  after  Alessandro 
Volta,  an  Italian  physicist  who  lived  from  1745  to  1827. 

229.  What  is  the  international  coulomb? 

"The  unit  of  quantity  shall  be  what  is  known  as  the  international 
coulomb,  which  is  the  quantity  of  electricity  transferred  by  a  current 
of  one  international  ampere  in  one  second."  (See  also  No.  137.) 
This  unit  is  named  after  Charles  A.  Coulomb,  a  celebrated  French 
mathematical  physicist,  who  lived  from  1736  to  1806. 

230.  What  is  the  international  farad ? 

"The  unit  of  capacity  shall  be  what  is  known  as  the  international 
farad,  which  is  the  capacity  of  a  condenser  charged  to  a  potential  of 
one  international  volt  by  one  international  coulomb  of  electricity." 
(See  also  No.  132.)  This  unit  is  named  after  Michael  Faraday,  a 
celebrated  English  physicist,  who  lived  from  1791  to  1867. 

231.  What  is  the  international  joule? 

"This  unit  work  shall  be  the  joule,  which  is  equal  to  10,000,000 
units  of  work  in  the  centimeter-gram-second  system,  and  which  is 
practically  equivalent  to  the  energy  expended  in  one  second  by  one 
international  ampere  in  an  international  ohm."  (See  also  No.  213.) 

232.  What  is  the  international  watt? 

"The  unit  of  power  shall  be  the  watt,  which  is  equal  to  10,000,000 
units  of  power  in  the  centimeter-gram-second  system,  and  which  is 
practically  equivalent  to  the  work  done  at  the  rate  of  one  joule  per 
second."  (See  No.  213.) 

233.  What  is  an  international  henry? 

"The  unit  of  induction  shall  be  the  henry,  which  is  the  induction 
in  a  circuit  when  the  electromotive  force  induced  in  this  circuit  is 
one  international  volt,  while  the  inducing  current  varies  at  the  rate 
of  i  ampere  per  second."  This  unit  was  named  after  Joseph  Henry, 
an  American  physicist,  who  discovered  many  of  the  laws  of  electro- 
magnetic induction. 

234.  What  is  meant  by  a  legal  ohm? 

The  "legal  ohm"  was  the  temporary  standard  of  resistance  adopted 
by  a  committee  of  the  International  Congress  of  Electricians,  held  at 


58  ELECTRICAL   CATECHISM. 

the  Paris  Exhibition  in  1881,     This  was  the  standard  from  1882  un- 

1060 
til  1894,  and  is  equal  to    — g —  international   ohms.       (See   Nos. 

226,234.) 

235.  What  is  meant  by  a  legal  volt? 

The  "legal  volt"  was  adopted  by  the  same  committee  and  bears 
a  similar  ratio  to  the  international  volt.  (See  also  Nos.  138,  228.) 

236.  What  is  the  legal  ampere? 

The  "legal  ampere"  had  the  same  value  as  the  international  am- 
pere. (See  No.  227.) 

237.  What  is  meant  by  the  "B.  A.  unit?" 

The  British  Association  made  a  determination  of  the  value  of  the 
ohm  in  1863  and  constructed  several  copies  of  their  standard.  This 
value  was  equal  to  0.9863  international  or  true  ohms.  (See  No. 
226.) 

238.  What  is  meant  by  "Siemens'  units?" 

The  "Siemens'  unit"  of  resistance  was  originally  proposed  by 
Werner  Siemens  (a  German  electrician  living  from  1816  to  1892), 
being  the  resistance  of  a  column  of  mercury  I  m.  long  and  I  sq.  mm. 
in  section.  This  was  in  use  for  a  number  of  years  in  Germany.  It 
equals  0.9408  international  ohms.  (See  No.  224.) 

239.  What  is  a  megohm? 

A  megohm  is  1,000,000  ohms.  It  is  a  unit  often  used  in  measuring 
very  high  resistances,  such  as  insulation.  For  instance,  when  a  wire 
is  guaranteed  to  have  an  insulation  resistance  of  not  less  than  300 
megohms  per  mile,  it  means  that  the  resistance  between  the  wire  and 
the  earth  or  the  lead  covering  is  not  less  than  300,000,000  ohms  for 
a  length  of  I  mile,  and  proportionally  greater  for  shorter  lengths. 

240.  What  is  a  microhm? 

A  microhm  is  a  unit  sometimes  used,  being  equal  to  one  millionth 
part  of  an  ohm.  For  example,  the  specific  resistance  of  substances 
is  usually  given  in  microhms,  being  the  resistance  in  millionths  of  an 
ohm  between  the  parallel  faces  of  a  cube  I  cm.  in  each  direction. 

241.  What  is  a  "maxwell?" 

The  maxwell  is  one  line  of  force,  the  practical  and  the  c.g.s.  unit  of 
magnetic  flux,  which  when  uniformly  distributed  over  a  cross  section 

of  i  sq.  cm.  produces  a  tension  of  -  -  dynes.     The  Paris  Congress  of 

8~ 

1900  named  it  after  James  Clerk  Maxwell  (1831-1879),  founder  of 
the  electromagnetic  theory  of  light. 


UNITS. 


59 


242.  What  is  a  "gauss?" 

The  name  "gauss"  was  adopted  by  the  Paris  International  Elec- 
trical Congress  of  1900  for  the  practical  unit  of  magnetic  intensity, 
being  one  maxwell,  or  one  line  of  force  per  square  centimeter.  The 
unit  is  named  after  Carl  Frederick  Gauss  (1777-1855),  who  invented 
the  declination  compass. 

243.  W hat  is  a  "line  of  force?" 

A  line  of  force  is  the  expression  used  to  indicate  either  the  direc- 
tion or  the  strength  of  a  magnetic  force.  The  "unit  magnet  pole" 
is  a  magnetic  pole  of  such  strength  that  it  will  repel  an  equal  pole 
with  a  force  of  I  dyne  when  at  a  distance  of  I  cm.  apart.  Any 
magnetic  field  in  which  such  a  unit  pole  is  attracted  or  repelled 
with  a  force  of  I  dyne  is  said  to  have  a  strength  of  "one  line  of  force 
per  square  centimeter." 

244.  How  strong  is  the  earth's  magnetic  field? 

It  varies  in  different  places  and  at  different  times.  In  New  York 
City,  the  total  magnetic  field  due  to  the  earth's  magnetism  has  a 
strength  of  about  0.61  line  of  force  per  square  centimeter,  in  the 
direction  of  greatest  force,  which  is  about  70  degrees  below  the 

3C=  0.208 


FIG.  244. 

horizontal.  The  force  in  the  horizontal  direction,  often  called 
"  horizontal  intensity  "  or  "  3C,"  is  0.208,  while  the  vertical  com- 
ponent is  0.573  gausses.  (See  also  No.  777.) 

245.  What  is  meant  by  "471-"  as  used  in  connection  with  elec- 
tricity or  magnetism? 

The  Greek  letter  TT  (pronounced  "pi")  is  a  character  adopted  by 
mathematicians  to  denote  the  relation  between  the  diameter  and  the 
circumference  of  a  circle,  and  equals  3.1416;  that  is,  the  circumfer- 


60  ELECTRICAL   CATECHISM. 

ence  of  a  circle  is  3.14  times  its  diameter.  The  area  of  a  sphere  is 
4  TT  (or  12.566)  times  its  radius  squared,  or  TT  times  its  diameter 
squared.  For  example,  if  the  diameter  of  a  circle  is  6  ins.,  its  cir- 
cumference is  3.14  x  6,  or  18.84  ms-  5  the  area  of  a  circle  6  ins.  in 
diameter  is  3.14  x  6  x  6,  or  113.1  sq.  ins. 

246.  What  does  TT  have  to  do  with  electricity? 

The  exact  relation  is  rather  complicated  and  mathematical,  but, 
in  general  terms,  it  comes  from  the  original  unit  taken  for  magnetic 
force.  At  unit  distance  from  a  unit  magnetic  pole,  the  field  has 
unit  strength,  and  there  is  one  line  of  force  per  square  centimeter; 
but  this  force  is  equal  in  all  directions  from  the  unit  pole,  and  since 
a  sphere  with  radius  equal  to  unity  has  a  surface  of  4  TT,  it  follows 
that  there  are  4  TT  lines  of  force  coming  from  a  unit  pole.  In  a 
more  complicated  way,  the  magnetic  force  due  to  a  current  flowing 

through  a  coil  equals  -  -  times  the  product  of  amperes  by  turns  of 
wire.  (See  Nos.  136,  224.) 

247.  What  is  a  "gilbert ?" 

The  name  "gilbert"  has  been  proposed  for  the  practical  unit  of 

magnetomotive  force  and  equals  — — ampere  turn.    The  unit  is  named 

47T 

after  William  Gilbert  (1540-1603),  a  court  physician  in  England, 
who  made  a  careful  study  of  magnetism  and  wrote  the  first  extended 
treatise  on  the  subject. 

248.  What  is  an  ampere-turn? 

The  magnetizing  force  of  a  current  flowing  through  a  coil  varies  as 
the  product  of  the  current  by  the  number  of  turns  in  the  coil.  A 
current  of  I  amp.  through  a  coil  of  100  turns  would  have  the  same 
magnetic  effect  as  a  current  of  10  amps,  through  a  coil  of  ten 
turns,  provided  both  coils  were  of  similar  dimensions,  having  the 
same  diameter  and  length.  (If  any  iron  is  near,  it  must  be  assumed 
the  same  in  the  two  cases.)  In  each  case  the  magnetizing  force  is  4?r 
times  100  amp. -turns,  or  1256.6  gilberts. 

249.  What  is  reluctance? 

Reluctance  in  a  magnetic  circuit  corresponds  closely  with  resist- 
ance in  an  electric  circuit.  The  law  for  magnetic  circuits  is  very 
similar  to  that  for  electric  circuits.  The  magnetic  flux  (correspond- 
ing to  current)  equals  the  magnetomotive  force  (corresponding  to 
voltage  or  electromotive  force)  divided  by  reluctance. 


UNITS.  61 

250.  What  is  the  "oersted?'' 

"Oersted"  has  been  proposed  for  the  name  of  the  practical  unit  of 
reluctance,  being  such  as  to  permit  one  maxwell  to  traverse  it  under 
the  action  of  a  magnetomotive  force  of  one  gilbert.  The  unit  is 
named  after  Hans  Christian  Oersted  (1777  to  1851),  a  Danish 
physicist,  who  discovered  the  action  of  a  current  on  an  electromagnet. 

251.  Are  there  other  electrical  units  than  those  noted  above? 

Other  units,  which  are  not  so  fundamental  as  those  already  dis- 
cussed, are  considered  in  later  numbers ;  the  mho  in  No.  335,  the 
circular  mil  in  No.  341,  the  square  mil  in  No.  342,  the  mil-foot  in 
No.  344,  the  mile-ohm  in  No.  357. 

For  tabular  statement  of  relations  between  various  units,  see  Ap- 
pendix II. 


CHAPTER  III. 

LAWS  OF  ELECTRIC  CIRCUITS. 

300.  How  many  kinds  of  current  are  there? 

Currents  may  be  classified  on  the  basis  of  directionality  into  uni- 
directional (or  direct)  and  alternating.  The  latter  may  be  divided  in- 
to harmonic  and  irregular.  Unidirectional  currents  may  be  divided 
into  constant  (or  continuous),  pulsating  and  irregular  currents. 
Sometimes  currents  are  classified  into  galvanic  (or  voltaic)  and  in- 
duced (or  faradic)  currents. 

301.  What  are  voltaic  or  galvanic  currents? 

This  is  a  name  used  by  physicians  for  unidirectional  steady  cur- 
rents, such  as  given  by  a  battery  to  a  circuit  of  steady  resistance. 
When  applied  to  patients  they  call  it  galvanization. 

302.  What  is  meant  by  franklinization? 

This  medical  term  is  used  for  the  application  of  static  electricity, 
that  is,  electricity  of  very  high  pressure  and  small  quantity. 

303.  What  is  meant  by  faradization? 

This  is  a  medical  term  referring  to  the  application  to  patients  of 
interrupted  or  induced  currents. 

304.  What  is  a  sinusoidal  current? 

A  sinusoidal  current  is  a  common  variety  of  alternating  current  in 
which  the  current  gradually  increases  from  zero  to  a  maximum  value, 


FIG.  305.— SINE  CURVE. 


then  becomes  weaker  until  it  reaches  zero,  then  changes  direction 
and  rises  gradually  to  a  maximum  and  so  on.     (See  No.  1401.) 


ELECTRIC   CIRCUITS.  63 

305.  What  is  a  pulsating  current? 

A  pulsating  current  goes  always  in  the  same  direction,  but  changes 
its  strength,  not  necessarily  falling  to  zero,  but  rising  and  falling  by 
more  or  less  regular  gradations. 

306,  What  is  an  interrupted  current? 

One  that  has  nearly  constant  strength,  but  one  that  is  off  and  on 
at  regular  intervals. 

307  Is  there  any  difference  between  the  current  obtained  from 
a  dynamo  and  that  from  a  battery  of  equal  voltage? 

Current  from  a  battery  is  steady,  while  there  is  a  slight  pulsation 
in  the  current  obtained  from  a  dynamo, 'even  if  it  be  a  direct-current 
machine. 

308.  Is  there  any  analogy  between  the  How  of  electricity  and  the 
How  of  water? 

The  number  of  amperes  in  an  electric  circuit  corresponds  closely 
to  the  number  of  cubic  feet  or  gallons  per  minute  flowing  through 
a  pipe.  The  electric  pressure  or  voltage  corresponds  closely  to  the 
number  of  pounds  pressure  per  square  inch.  The  electrical  resist- 
ance corresponds  to  some  extent  to  the  friction  along  the  surface  of 
the  pipes,  the  difference  being  that  the  resistance  to  the  flow  of  water 
depends  upon  both  the  surface  of  the  pipe  and  upon  its  area.  The 
fall  of  potential  or  "drop"  due  to  the  passage  of  current  through  a 
conductor  equals  the  product  of  current  by  resistance,  corresponding 
to  the  loss  of  pressure  in  a  pipe  proportional  to  the  surface  friction 
of  the  pipe  arid  to  the  amount  of  water  flowing.  The 
pump  and  the  dynamo  are  analogous,  while  the  water  motors  cor- 
respond to  the  electric  motors  or  lamps.  Sometimes  water  motors 
are  arranged  tandem,  so  that  the  same  water  passes  through  two  or 
more ;  such  an  arrangement  corresponds  closely  to  a  "series"  elec- 
tric circuit.  Generally  the  water  motors  are  arranged  so  that  each 
takes  its  own  water  and  receives  the  full  pressure  on  the  mains ;  this 
corresponds  closely  with  "multiple  arc"  electric  circuits  such  ,as 
used  for  motors  or  incandescent  lamps. 

309.  Is  there  any  definite  direction  to  the  How  of  an  electric  cur- 
rent? 

The  magnetic  and  chemical  effects  of  the  current  show  a  direction- 
ality. Scientists  have  agreed  to  say  that  a  current  flows  in  one  di- 
rection when  certain  effects  appear,  and  to  say  it  flows  in  the  op- 
posite direction  when  these  results  are  reversed ;  but  it  is  not  certain 
that  what  is  called  positive  may  not  actually  be  negative.  By  com- 
mon consent,  a  current  acting  in  a  certain  manner  is  said  to  flow 


64  ELECTRICAL   CATECHISM. 

in  a  given  direction,  the  point  from  which  it  flows  being  called  posi- 
tive, and  that  to  which  it  flows  being  called  negative.  This  is  somer 
what  analogous  to  what  we  call  down  and  up  in  relation  to  gravity, 
although  it  is  not  absolutely  certain  whether  a  body  falls  to  the 
ground  because  it  is  pulled  by  the  earth  or  whether  it  is  pushed  by 
some  force  from  outside. 

310.  How  can  the  direction  of  a  current  be  determined  from  its 
magnetic  effects? 

If  the  current  is  of  considerable  strength,  its  direction  may  be 
shown  by  a  compass  needle,  or  by  a  small  magnet  suspended  by  a 
thread  or  fine  wire.  Place  the  conductor  so  that  its  direction  is  ap- 
proximately north  and  south,  hold  the  magnet  or  compass  just 
above  the  conductor  and  let  the  observer  face  the  north.  If  the 
north-pointing  end  of  the  magnet  is  deflected  to  the  right,  the  cur- 
rent is  going  away  from  the  observer ;  while  if  the  south-pointing 
end  is  deflected  to  the  right,  the  current  is  coming  toward  the  ob- 
server. 

Another  method  is  to  wind  the  conductor  a  few  times  around  an 
iron  rod  and  note  the  end  which  attracts  the  north-pointing  end  and 
repels  the  south-pointing  end  of  the  magnet.  The  current  flows 
clockwise  around  the  coil  as  one  looks  from  the  attracting  pole 
toward  the  other. 

311.  How  can  the  direction  of  the  current  be  determined  by  its 
chemical  effect? 

In  a  glass  containing  water  acidulated  with  sulphuric  acid  (ten 
parts  water,  one  part  acid),  place  two  pieces  of  clean  lead  and  con- 
nect the  same  to  the  two  poles  of  the  dynamo,  putting  an  incandescent 
lamp  in  series  as  suggested  in  the  figure.  The  surface  of  the  lead 


•o- 


FIG.  311.— POLARITY  TEST. 


connected  to  the  positive  pole  will  become  reddish  brown  after  the 
current  has  passed  for  a  short  time,  and  the  surface  of  the  other 
piece  will  assume  a  grayish  color.  This  method  may  be  applied 


ELECTRIC   CIRCUITS.  65 

to  dynamos  for  incandescent  lamps  and  for  charging  storage  bat- 
teries, but  is  not  suitable  for  arc  machines  except  by  changing  the 
connections.  For  testing  the  direction  of  current  from  an  arc 
dynamo,  the  lead  cell  with  the  incandescent  lamp  may  be  connected 
in  shunt  around  an  arc  lamp.  The  simplest  method,  however,  of 
testing  the  direction  of  current  from  an  arc  dynamo  is  to  notice  the 
carbons  immediately  after  the  current  has  been  cut  off  from  an  arc 
lamp,  when  the  positive  carbon  will  be  found  to  be  much  hotter  than 
the  negative  carbon. 

312.  What  solution  is  used  for  moistening  the  paper  in  polarity 
indicators  ? 

The  simplest  solution  is  iodide  of  potassium  dissolved  in  water; 
when  paper  is  moistened  with  this,  and  the  terminals  of  a  battery 
or  dynamo  are  connected  to  it,  a  brown  spot  is  left  at  the  positive 
terminal,  or  where  the  current  enters ;  the  paper  must  be  damp  when 
the  test  is  applied.  Adding  glycerine  to  the  solution  keeps  the  paper 
moist  longer.  By  moistening  paper  with  a  solution  of  iodide  of 
potassium,  to  which  some  starch  water  has  been  added,  a  blue  mark  is 
left  at  the  positive  pole.  Another  solution  is  made  by  dissolving  15 
grains  of  red  iodide  of  mercury  and  20  grains  of  iodide  of  potassium 
in  one  fluid  ounce  of  glycerine ;  this  must  be  used  moist,  keeping  the 
terminals  I  in.  apart  for  each  100  volts ;  a  yellow  mark  is  left  at  the 
negative  pole.  By  using  a  solution  of  ferrocyanide  of  potassium 
and  iron  terminals,  a  blue  mark  is  left  at  the  positive  pole. 

In  applying  any  of  these  tests,  it  is  desirable  to  avoid  all  risk  of 
making  a  short  circuit.  For  this  reason  it  is  common  to  place  an  in- 
candescent lamp  in  series  with  the  test  paper. 

313.  What  is  the  potato  test  for  polarity? 

A  convenient  test  is  to  stick  the  ends  of  two  wires  into  a  potato, 
keeping  the  ends  an  inch  or  more  apart.  When  the  wires  are  con- 
nected to  a  charged  circuit,  the  potato  will  boil  at  the  negative  ter- 
minal on  account  of  the  gas  set  free  by  chemical  decomposition. 

314.  Why  do  electricians  attach  so  much  importance  to  Ohm's 
law? 

Because  it  is  the  basis  of  most  calculations  relating  to  electricity. 
It  is  a  very  simple  law,  but  its  applications  are  often  quite  complex 
and  difficult  to  follow. 

315.  What  is  Ohm's  law. 

"The  strength  of  the  current  varies  directly  as  the  electromotive 
force,  and  inversely  as  the  resistance  of  the  circuit,"  is  the  form  in 


66  ELECTRICAL   CATECHISM. 

which  it  was  originally  stated  by  its  discoverer,  George  Simon  Ohm, 
a  German  electrician  who  lived  from  1789  to  1854.  Using  the  units 
adopted  by  modern  electricians,  the  law  is  :  "The  number  of  amperes 
of  current  flowing  through  a  circuit  is  equal  to  the  number  of  volts 
of  electromotive  force  divided  by  the  number  of  ohms  of  resistance. 
This  may  be  written, 

Current  (in  amperes)  = 

Electromotive  force  (in  volts) 

Resistance  (in  ohms) 

316.  Is  there  a  shorter  statement  of  Ohm's  law? 

"Current  equals  pressure  divided  by  resistance,"  or  "amperes 
equal  volts  divided  by  ohms."  A  still  shorter  form  is  to  write  simply 
the  initial  letters  from  above  equation, 

E  E 

C  =  — ,  or  I  =  — . 

R  R 

Although  "C"  is  the  natural  abbreviation  for  curreat,  the  standard 
practice  is  to  use  the  letter  "I"  to  represent  current. 

317.  What  is  an  ampere? 

The  ampere  (named  after  Andre  Marie  Ampere,  a  French  physi- 
cist who  lived  from  1775  to  1836)  is  the  unit  of  current.  Defined  in 
terms  of  other  units,  the  ampere  is  the  strength  of  current  necessary 
to  carry  one  coulomb  (see  Nos.  137  and  229)  in  one  second.  It  is 
the  current  which  an  electromotive  force  of  one  volt  (see  Nos.  138 
and  228)  will  send  through  a  circuit  whose  resistance  is  one  ohm 
(see  Nos.  226  and  234).  It  is  the  current  which,  passing  through 
I  cm.  of  wire  bent  into  part  of  a  circle  of  I  cm.  radius,  would  attract 
or  repel  a  unit  magnet  pole  at  the  center  of  the  circle  with  a  force 
equal  to  one-tenth  dyne.  It  is  one-tenth  of  the  "absolute"  or  "C.  G. 
S."  unit  of  current.  It  is  a  current  which  will  deposit  silver  from  a 
standard  solution  at  the  rate  of  0.001118  gram  per  second.  (See 
also  No.  227.) 

318.  What  is  an  ohm? 

The  ohm  is  the  unit  of  electrical  resistance.  Theoretically,  it  is 
a  velocity  of  one  earth-quadrant  per  second;  that  is,  the  speed  re- 
quired to  travel  from  the  earth's  equator  to  its  pole  in  one  second. 
(For  the  practical  ohm,  see  Nos.  226  and  234.) 

319.  How  is  Ohm's  law  used  in  electrical  calculations? 

If  two  of  the  three  quantities  are  known,  the  other  can  be  calcu- 
lated easily  by  simple  rules  of  arithmetic. 


ELECTRIC    CIRCUITS.  67 

E 

320.     Does  Ohm's  law  take  other  forms  than  I  =  ^  ? 

E 
It  may  be  written  E  —  I  R  or  R  =  — 

To  illustrate,  suppose  in  an  electrical  circuit  in  which  5  amp.  are  flow- 
ing that  the  E.M.F.  is  ten  volts,  and  that  the  entire  resistance  of 
the  circuit  is  two  ohms.  Then  we  have 

E_io_ 
~-~  = 


E  =  I  R  =  5  x  2  =  10. 

321  How  much  current  will  100  volts  send  through  a  circuit 
of  5  ohms? 

Since  current  equals  voltage  divided  by  resistance,  the  current  in 
this  case  is  100  divided  by  5,  or  20  amp. 

322.  How  much  electromotive  force  is  necessary  to  send  10  amp. 
through  4.5  ohms? 

Since  voltage  equals  current  multiplied  by  resistance,  it  requires 
10  times  4.5,  or  45  volts. 

323.  Through  what  resistance  will  50  volts  send  2  amp.? 
Resistance  equals  voltage  divided  by  current.     Hence,  in  this  case, 

resistance  equals  50  divided  by  2,  or  25  ohms. 

324.  What   is   the   resistance   of   a    i6-cp.    incandescent    lamp, 
through  which  no  volts  send  a  current  of  1-2  amp.? 

Since  resistance  equals  voltage  divided  by  current,  the  resistance 
of  the  lamp  is  no  divided  by  0.5,  or  220  ohms. 

325.  Does  Ohms  law  hold  true  for  every  part  of  a  circuit,  as  well 
as  for  the  whole  circuit  ? 

It  does  when  properly  applied.  The  pressure  required  to  send 
current  through  any  part  of  a  circuit,  equals  the  product  of  the  cur- 
rent by  the  resistance  of  that  part  of  the  circuit. 

326.  How  does  Ohm's  law  apply  when  there  are  several  parts 
in  a  circuit? 

The  total  current  equals  the  total  electromotive  force  divided  by 
the  total  resistance.  There  may  be  several  currents,  several  electro- 
motive forces  and  several  resistances  in  the  same  circuit.  The  con- 
ditions of  each  circuit  must  be  considered  before  applying  the  law. 


63  ELECTRICAL   CATECHISM. 

327.  What  is  a  series  circuit? 

A  series  circuit  is  one  consisting  of  several  parts  connected  "in 
series"  or  in  a  row,  so  that  the  same  current  passes  through  each  part 
one  after  the  other. 

328.  Give  an  example  where  several  resistances  are  in  series. 

A  telegraph  line  is  a  good  example,  the  circuit  consisting  of  th? 
line  wire,  the  keys  and  relays,  the  batteries  and  the  earth  return. 


FIG.  328.—  TELEGRAPH  CIRCUIT. 

Suppose  the  line  consists  of  .100  miles  of  No.  8  iron  wire  having 
a  resistance  of  12.6  ohms  per  mile,  15  relays  of  150  ohms  each,  two 
batteries  of  50  cells  of  4  ohms  each,  and  the  earth  re- 
turn with  a  resistance  of  25  ohms.  The  resistance  of 
the  line  is  100  times  12.6,  or  1260  ohms;  that  of  the 
relays  is  15  times  150,  or  2250  ohms;  that  of  the  batteries  is  50 
times  4,  or  200  ohms  for  each  battery,  or  400  ohms  for  the  two.  The 
total  resistance  of  the  circuit,  allowing  25  ohms  for  resistance  of  keys 
and  contacts,  is  the  sum  of  1260  plus  2250  plus  400  plus  25  plus  25, 
which  equals  3960  ohms,  or, 

Rr=  (12.6  X  100)  +  (150  X  15)  +  (4X50X2)  +25  +  25. 

R  =  1260  +  2250  +  400  +  25  +  25. 

R  =  3960  ohms. 

329.  How  can  the  current  in  such  a  line  be  calculated? 

The  batteries  give  an  average  of  about  1.07  volts  per  cell,  hence 
the  100  cells  give  an  electromotive  force  of  107  volts.  The  current 
equals  voltage  divided  by  resistance,  or  107  divided  by  3960,  which 
equals  0.027  amperes,  or  27  milli-amperes. 

330.  'Are  series  circuits  used  in  electric  lighting? 

Arc  and  incandescent  lamps  for  street  lighting  are  generally  in 
series  circuits  supplied  with  current  of  constant  strength,  either 
direct  or  alternating.  For  example,  a  direct  current  open  arc  lamp, 
such  as  formerly  used  for  street  lighting,  required  50  volts  with  6.8 
or  9.6  amperes,  thus  having  an  effective  resistance  of  about  7.5  or 
5.2  ohms.  For  9.6  amperes,  the  line  is  made  of  No.  6  copper  wire, 


ELECTRIC .  CIRCUITS. 


69 


having  a  resistance  of  about  2.1  ohm  per  mile.     A  circuit  15  miles 
long  with  60  open  arc  lamps  with  9.6  amperes  has  a  resistance  equiv- 


FIG.  330.—  SERIES  ARC  LIGHT  CIRCUIT. 

alent  to  about  60  times  5.2  ohms  for  the  lamps,  plus  15  times  2.1 
ohms  for  the  line,  or  312  plus  31.5,  or  343.5  ohms;  or 

R=  (6oX5-2)  +  (15X2.1)  =312  +  31.5  =  343-5  ohms- 
The  voltage  at  the  dynamo  necessary  to  send  9.6  amperes  through 
the  circuit  is  about  9.6  times  343-5,  or  about  3298  volts5  or 
E  =  I  R  =  9.6  X  343-5  =  3298  volts. 

331.  What  is  a  multiple  circuit  f 

A  multiple  circuit  is  one  that  is  branched  so  that  the  current  di- 
vides in  several  paths.  Such  a  circuit  may  be  called  "multiple  arc," 
"multiple,"  "parallel,"  "branched,"  or  "divided."  The  parts  may  be 
said  to  be  connected  "in  multiple,"  "in  parallel,"  "in  multiple  arc," 
"in  derivation,"  or  "in  shunt."  (See  Figs.  638,  901,  1146,  1410.) 

332.  What  is  the  resistance  of  a  multiple  circuit? 

It  is  obtained  most  easily  by  considering  the  conductivity,  which  is 
the  reciprocal  of  the  resistance.  The  conductivity  of  the  whole 
equals  the  sum  of  the  conductivities  of  the  parts,  hence  the  resistance 
of  the  whole  equals  the  reciprocal  of  the  sum  of  the  conductivities, 
or  the  resistance  of  the  whole  equals  the  reciprocal  of  the  sum  of  the 
reciprocals  of  the  resistances  of  the  separate  parts.  For  a  circuit 
of  three  branches,  this  may  be  expressed  in  a  formula  as  follows  : 


K       K'  +  K"  +  K'" 


r  r+  r  r 


r  r 


i       i       i 

—  ;  +  —  „  +  *-n, 

r       r       r 

One  way  of  expressing  the  joint  resistance  is  to  say  that  the  re- 
sistance of"  a  branched  circuit  equals  the  product  of  all  the  parts,  di- 
vided by  the  sum  of  the  products  of  each  part  by  every  other  part 


70  ELECTRICAL   CATECHISM. 

except  one.  When  the  parts  are  equal,  the  formula  reduces  to  a 
simple  form,  and  the  joint  resistance  equals  that  of  one  part  divided 
by  the  number  of  equal  parts. 

333-     Give  an  example  of  a  branched  circuit  with  equal  parts. 

Take  a  circuit  with  ten  i6-cp  no-volt  incandescent  lamps  in 
multiple.  The  resistance  of  one  lamp  is  about  220  ohms ;  the  re- 
ristance  of  the  group  is  220  divided  by  10,  or  22  ohms.  This  can 
be  checked  easily :  the  resistance  of  one  lamp  being  220,  it  takes  1 10 
divided  by  220,  or  0.5  amp. ;  each  lamp  takes  an  equal  amount,  so 
the  ten  lamps  would  take  10  times  0.5,  or  5  amp. ;  by  Ohm's  law, 
when  the  current  is  5  and  the  voltage  no,  the  resistance  is  22  ohms. 

334.  Give  an  example  of  a  branched  circuit  ivith  unequal  parts. 
An  incandescent  lighting  circuit  divides  into  three  circuits,  having 

five,  eight  and  ten  lamps,  respectively.  The  resistances  of  these 
branches  are  therefore  44,  27.5  and  22  ohms  ;  their  conductivities  are 
0.0228,  0.0364  and  0.0455  "mhos ;"  the  sum  of  their  conductivities  is 
0.10454  mho,  and  the  reciprocal  of  this  sum  is  9.56  ohms.  This  may 
be  checked  as  follows:  The  first  branch  takes  no  divided  by  44,  or 
2.5  amp. ;  the  second  branch  takes  4  amp.,  and  the  third  branch 
5  amp.,  making  a  total  of  11.5  amp.;  to  take  this  current,  the  re- 
sistance is  no  divided  by  11.5,  or  9.55  ohms. 

335.  What  is  a  "mho?" 

The  mho  is  the  unit  of  conductivity  and  is  the  reciprocal  of  the 
conductivity.  For  example,  a  conductor  with  resistance  of  5  ohms 
has  a  conductivity  of  1-5  or  0.2  mho. 

336.  What  are  series-multiple  or  multiple-series  circuits? 
Practice  is  somewhat  divided  in  the  use  of  these  words.      With 

some,  a  multiple-series  circuit  means  a  circuit  consisting  of  a  mul- 
tiple of  series  circuits ;  others  mean  multiple  circuits  in  series.  The 
two  are  suggested  in  the  accompanying  figure.  Examples  of  mul- 


FIG.  336.— SERIES  MULTIPLE  AND  MULTIPLE  SERIES  CIRCUITS.  « 

tiple-of-series  circuits  are  seen  in  the  lighting  circuits  of  street  cars, 
in  which  five  incandescent  lamps  are  connected  in  series,  and  sev- 
eral of  these  circuits  are  connected  in  multiple  across  the  5oo-volt 
line.  Series-of-multiple  circuits  were  formerly  used  for  operating 


ELECTRIC   CIRCUITS.  71 

incandescent  lamps  on  arc  circuits,  the  current  being  divided  among 
several  lamps,  so  that  each  lamp  took  only  part  of  the  whole  current ; 
these  systems  proved  to  be  sources  of  fire  risk,  and  have  been  aban- 
doned. 

337.  Are  multiple-series  circuits  used  for  any  purpose  except 
for  lighting  from  electric  railway  circuits? 

Incandescent  electric  lamps  for  street  lighting  purposes  are  ccm- 
monly  operated  on  multiple-series  circuits.  The  "municipal  system," 
formerly  sold  by  the  Edison  Company,  has  a  direct-current  dynamo 


FIG.    337A.— EDISON    MUNICIPAL    SYSTEM. 

giving  about  1200  volts.  A  "string"  of  twenty  or  more  incandescent 
lamps  taking  4  amp.  was  connected  in  series  with  an  adjustable 
lamp  board  or  "bank  board,"  having  an  ammeter  and  a  number  of 
lamps  which  could  be  switched  into  the  circuit  when  needed.  Sev- 
eral strings  of  lamps  were  connected  to  one  dynamo,  enough  lamps 
being  in  each  circuit  so  that  the  1200  volts  would  send  the  required 
4  amperes  through  the  circuit.  With  alternating  currents  the  out- 
side and  inside  lamps  are  operated  from  the  same  generators,  and 
even  from  the  same  circuits,  as  indicated  in  Fig.  33?B,  in  which  the 
street  lamp  current  is  controlled  by  cutting  in  more  or  less  lamps 
at  the  station.  The  Westinghouse  Company  shunts  each  lamp  by  a 
coil  which  normally  takes  little  current,  but  which  carries  the  whole 
current  when  the  lamp  breaks,  and  substitutes  a  counter-electro- 


ELECTRICAL  CATECHISM. 


FIG.  337B.— ALTERNATING  CURRENT  SYSTEM  WITH  SERIES  INCAN- 
DESCENT STREET  LAMPS  AND  MULTIPLE  COMMERCIAL  LAMPS 


FIG.  337c.— WESTINGHOUSE  INCANDESCENT  SERIES  STREET  LIGHTING 

SYSTEM. 


ELECTRIC  CIRCUITS. 


73 


motive  force  for  the  ohmic  drop  through  the  lamp  (see  Nos.  361  and 
1477).  The  late  practice  is  to  use  special  transformers  giving  con- 
stant current  and  insulating  the  street  circuit. 


AfyXkTf 

Arnrrtcter 


OpcnCircu, 
P/tsff  vW/CC 
Constant  Current 
Transformer 


Primary  Plug  Snitches  © 


Primary  } 


FIG.  337D.— GENERAL  ELECTRIC  INCANDESCENT  SERIES  STREET  LIGHTING 

SYSTEM. 

338.  What  is  the  difference  between  a  conductor  and  a  resistance? 
It  is  a  question  of  degree  and  purpose.     No  substance  is  a  perfect 

conductor,  therefore  every  conductor  offers  more  or  less  resistance 
to  a  current.  If  one  wishes  to  increase  the  resistance  of  a  circuit, 
he  puts  in  series  a  conductor  of  the  desired  resistance.  If  he  de- 
sires to  reduce  the  resistance,  that  is,  to  increase  the  conductivity  of 
the  circuit,  the  conductors  are  replaced  by  others  of  higher  con- 
ductivity, or  several  conductors  are  connected  in  multiple. 

339.  What  determines  the  resistance  of  a  conductor? 

The  resistance  increases  directly  as  the  length,  inversely  as  the 
sectional  area,  and  depends  upon  the  "specific  resistance"  of  the 
material  of  which  the  conductor  is  composed. 

340.  What  is  meant  by  specific  resistance? 

The  specific  resistance  is  commonly  taken  as  the  resistance,  in 


ELECTRICAL   CATECHISM. 


millionths  of  an  ohm,  between  the  parallel  faces  of  a  cube  of  material 
I  cm.  in  each  dimension.   In  practical  engineering  the  resistance  of  a 
mil-foot  is  often  taken  as  the  unit.     Values  for  substances  commonl) 
used  as  conductors  are  given  in  the  following  table. 
SPECIFIC  RESISTANCE  OF  CONDUCTORS 


Conductor 

Specific  Resist- 
ance (Microhms 
Per  Cubic 
Centimeter) 

RESISTANCE  OF  ONE 
MIL-  FOOT. 

Temperature 
Coefficient 
per  Degree 
Centigrade 

Percent- 
age Con- 
ductivity 

0  Cent. 
32  Fahr. 

24  Cent. 
75  Fahr. 

1-594 
1.50 
9-75 

9-59 

9.4 
58.6 

17.75 
115.5 
600. 
126. 

10.507 

10.  16 
65-3 
78-5 
90.8 
19.4 
129. 
613. 
127.2 

.00388 
.00380 
.00463 

100. 
1  06. 

16.2 

135 

n.6 

54-5 

8.2 

1-73 
8-3 

Silver 

"EBB"  iron  

"BB"  iron. 

Steel  (wire)  

.0039 
.00387 
.00089 
.0004 

Aluminum      

2.889 
19.63 
94.07 
20.76 

Lead  

Mercury  

German  silver  .  . 

The  resistance  at  any  temperature  (above  zero)  equals  the  re- 
sistance at  zero  multiplied  by  one  plus  the  "temperature  coefficient" 
times  the  temperature  (in  Centigrade  degrees).  This  is  expressed 
by  the  formula, 

Rt  =  Ro  (i  +  «t). 

341.  What  is  a  circular  mil? 

The  circular  mil  is  the  unit  used  in  measuring  the  area  of  con- 
ductors. It  is  the  area  of  a  circle  one  mil,  that  is,  one  one-thousandth 
of  an  inch  in  diameter. 

342.  What  is  a  square  mil? 

The  square  mil  is  sometimes  used  as  a  unit  of  area,  being  the  area 
of  a  square  one  one-thousandth  of  an  inch  on  each  side. 

343.  What  is  the  relation  between  circular  mils  and  square  mils? 
The  number  of  circular  mils  multiplied  by  0.7854  gives  the  number 

of  square  mils.  Square  mils  multiplied  by  1.273  equals  circular  mils. 
These  figures  give  the  relation  between  the  area  of  a  circle  and  the 
surrounding  square. 

344.  What  is  a  mil-foot? 

The  mil-foot  is  a  wire  of  cylindrical  section,  having  a  diameter  of 
I  mil  (that  is,  one  one-thousandth  of  an  inch)  and  i  ft.  long.  The 
resistance  of  a  mil-foot  of  pure  copper  at  the  temperature  of  melting 
ice  is  9.59  ohms ;  it  weighs  0.000003027  pounds. 


ELECTRIC   CIRCUITS, 


75 


345.  Is  the  resistance  of  a  conductor  affected  by  the  temperature? 
The  resistance  of  metal  conductors  increases  about  four-tenths 

of  I  per  cent  for  each  degree  Centigrade  rise  in  temperature,  or 
about  twenty-two-hundredths  of  I  per  cent  for  I  deg.  F.  The  re- 
sistance of  carbon,  liquids  and  a  few  alloys  decreases  with  rise  of 
temperature.  German  silver,  manganin,  "  IA  IA,"  etc.,  change  little. 

346.  Is  resistance  affected  by  pressure? 

The  resistance'  of  a  solid  continuous  conductor  is  not  affected  by 
pressure.  The  resistance  of  a  conductor  containing  loose  connections, 
such  as  a  powder  or  a  pile  of  plates  lying  loosely  upon  one  another,  is 
diminished  by  pressure,  since  the  parts  are  brought  closer  together. 
The  resistance  at  the  surface  of  carbon  is  more  sensitive  to  pressure 
than  that  of  other  substances,  and  this  has  been  usefully  applied  in 
certain  forms  of  carbon  rheostat,  also  in  the  telephone  transmitter. 

347.  How  is  the  variable  resistance  of  carbon  utilized  in  the  tele- 
phone transmitter? 

This  may  be  illustrated  from  the  Runnings  transmitter,  which  is 
the  basis  of  most  modern  transmitters.  In  the  accompanying  figure, 


METAL  CASING 

FIG.  347.— TELEPHONE  TRANSMITTERS. 

C  represents  granulated  carbon,  which  is  held  between  a  solid  back, 
B,  and  the  diaphragm,  A}  so  that  the  circuit  between  the  binding 
posts,  EE,  is  through  the  back,  B,  the  carbon  particles,  C,  and  the 
diaphragm  A.  When  a  person  speaks  in  front  of  the  transmitter,  the 
diaphragm  is  thrown  into  vibration  and  thus  presses  against  the 
carbon  particles  more  or  less,  and  so  varies  the  resistance,  and  con- 
sequently the  current  varies  with  each  vibration  of  the  voice.  The 
figure  shows  also  an  improved  modern  form. 


76  ELECTRICAL  CATECHISM. 

348.  How  can  the  resistance  of  a  wire  be  determined? 

The  simplest  way  is  to  test  its  size  by  a  wire  gage  and  find  the 
resistance  from  a  table.  Instead  of  using  a  wire  gage,  the  diameter 
may  be  measured  by  a  micrometer,  and  the  resistance  may  then  be 
calculated  or  may  be  taken  from  a  table. 

349.  How  can  the  circular  mils  in  a  wire  be  calculated  from  its  di- 
ameter? 

When  the  section  of  a  wire  is  circular,  the  area  equals  the  diameter 
squared.  By  geometry  it  is  shown  that  the  areas  of  circles 
are  to  each  other  as  their  diameters  squared.  Taking  the  area  of  a 
circular  mil  as  the  unit,  the  area  of  any  round  wire  equals  the  square 
of  its  diameter  measured  in  thousandths  of  an  inch;  thus  the  area 
of  a  wire  having  a  diameter  of  102  thousandths  of  an  inch  is  102  X 
1 02,  or  10404  circ.  mils. 

350.  How  can  one  calculate  the  number  of  circular  mils  in  a  bar 
of  rectangular  section,  such  as  a  bus-barf 

It  is  most  easily  calculated  by  remembering  that  I  sq.  in.  equals 
1,273,000  circ.  mils.  Therefore,  find  the  area  of  the  bar  in  square 
inches  and  multiply  by  1,273,000,  and  the  result  will  be  the  cross 
section  in  circ.  mils. 

351.  Hoiu  can  the  resistance  be  calculated  when  the  diameter  and 
length  are  known? 

The  resistance  of  a  wire  equals  the  resistance  of  a  circular  mil- 
foot  multiplied  by  the  length  in  feet  and  divided  by  the  number  of 
circular  mils  in  its  area,  or  divided  by  the  square  of  its  diameter  in 
thousandths  of  an  inch.  Thus,  the  resistance  of  a  copper  wire  .102 
in.  in  diameter  and  1000  ft.  long  is, 

10.5   X   1000 

=   i.oi  ohm. 

102  X   1 02 

In  practice  it  is  common  to  take  n  as  the  resistance  of  a  mil-foot, 
thus  allowing  for  variations  in  diameter,  conductivity  and  imper- 
fect connections.  The  rule  is  often  written  as  a  formula : 

—     1 i  X  length       _    22  X  distance      ^ 
diameter  squared      diameter  squared ' 


ELECTRIC  CIRCUITS.  77 

352.  How  can  the  size  be  determined  to  give  a  certain  resistance 
in  a  wire  of  a  certain  length? 

The  formula  in  No.  351  can  be  easily  changed,  showing  that  the 
circular  mils,  or  the  square  of  the  diameter  of  the  wire,  equals  the 
length  multiplied  by  the  resistance  per  mil-foot  divided  by  the  re- 
quired resistance  (see  also  Nos.  367  to  371),  or, 


353-     Give  an  example  showing  how  the  formula  is  used. 
A  wire  I  mile  long  is  required  to  have  a  resistance  of  2  ohms. 

ii   X  5280 
c.  m.  =  -  =  28000  circ.  mils  ; 

2 

this  is  a  little  larger  than  No.  6  B.  &  S.  G.  If  it  were  necessary  to 
make  the  line  have  the  exact  resistance  stated,  it  might  be  made 
partly  of  No.  6  and  partly  of  No.  5. 

354.     How  can  one  calculate  the  length  of  wire  necessary  to  give 
a  desired  resistance? 

The  length  equals  the  resistance  multiplied  by  the  square  of  the 
diameter  and  divided  by  the  resistance  of  a  mil-foot,  or, 

R  d2 


355.  What  must  be  the  length  of  a  German  silver  wire  0.016  in. 
in  diameter  to  make  200  ohms? 

The  resistance  of  a  mil-foot  of  German  silver  is  about  127  ohms, 
varying  to  some  extent  in  different  lots.  For  200  ohms  the  re- 
quired length  is, 

200  X  16  X  16 

L  =  -  —  403.1  ft. 
127 

356.  What  is  the  resistance  of  ipo  ft.  of  "EBB"  telephone  wire 
0.064  in.  in  diameter? 

Resistance  equals  the  resistance  of  a  mil-foot  multiplied  by  the 
length  and  divided  by  the  area,  or  by  the  square  of  the  diameter. 
The  resistance  of  "EBB"  iron  wire  is  65.3  ohms  per  mil-foot  at  75 
degs.  F.  Therefore, 

65.3  X  ioo       6530 
R  =  —  -  -:  —  =  --  ^  =  1.6  ohms. 
64  X  64        4096 

357.  What  is  a  mile-ohm? 

The  mile-ohm  is  a  term  used  in  connection  with  telegraph  and  tele- 
phone wire.  It  is  the  resistance  of  a  wire  I  mile  long  and  weighing 


78  . ELECTRICAL  CATECHISM. 

i  Ib.  The  mile-ohm  of  pure  iron  is  about  4000,  the  specifications 
of  the  Western  Union  Company  calling  for  not  more  than  4800,  the 
British  office  calling  for  not  more  than  5323.  The  "mile-ohm"  is  really 
an  abbreviation  for  "weight  per  mile-ohm,"  and  is  sometimes  called 
"pound-mile-ohm."  The  former  specifies  "the  electrical  resistance  of 
the  wire  in  ohms  per  mile,  at  a  temperature  of  68  degs.  F.,  must  not 
exceed  the  quotient  arising  from  the  dividing  the  constant  number 
4800  by  the  weight  of  the  wire  in  pounds  per  mile.  The  coefficient 
.003  will  be  allowed  for  each  degree  Fahrenheit  in  reducing  to  stand- 
ard temperature."  One  mile-ohm  of  copper  at  60  degs.  F.  equals 
859  international  ohms,  or  861  legal  ohms,  or  868.9  B.  A.  units. 

358.  What  wire  gages  are  in  common  use? 

That  in  most  common  use  in  America  is  the  Brown  and  Sharp 
gage,  often  abbreviated  to  "B.  &  S.  G.,"  and  frequently  called  Ameri- 
can gage.  Iron  wire  is  generally  made  according  to  the  Birming- 
ham wire  gage,  often  abbreviated  to  "B.  W.  G."  Other  gages  some- 
times used  are  the  Washburn  &  Moen,  Roebling,  and  the  New  British 
or  Standard  gage.  Wires  are  often  designated  by  their  diameters 
without  reference  to  any  gage.  Very  large  cables  or  bars  are  usually 
designated  by  their  diameter  or  area  in  mils.  (See  tables  on  pages 

79  and  92.) 

359.  Are  there  any  easy  rules  for  remembering  the  wire,  table? 
A  B.  &  S.  G.  wire  three  sizes  larger  than  another  has  half  its 

resistance,  twice  its  weight  and  twice  its  area.  A  wire  ten  sizes 
larger  than  another  has  one-tenth  its  resistance,  ten  times  its  weight 
and  ten  times  its  area.  The  relative  values  of  resistance  ( for  decreas- 
ing sizes)-^-and  of  weight  and  area  for  consecutive  sizes  are:  0.50, 
0.63,  0.80,  i.oo,  1.25,  i. 60,  2.00.  The  relative  diameters  of  alternate 
sizes  of  wire  follow  the  same  schedule.  No.  10  wire  has  a  diameter 
of  o.io  inch  (102  mils),  an  area  of  10,000  circ.  mils  (10,380),  a 
resistance  of  i  ohm  per  1000  feet  at  20  deg.  cent.  (68  F.)  and  weighs 
31  pounds  (31.4)  per  1000  feet.  No.  5  wire  weighs  100  pounds  per 
looo  feet.  The  safe  carrying  capacity  doubles  every  fourth  size. 

360.  Is  a  circuit  likely  to  have  more  than  one  electromotive  force? 
The  principal  E.M.F.  causing  current  to  flow  in  the  circuit  may 

be  in  one  or  more  sections,  which  may  be  in  one  or  more  parts  of  the 
circuit.  Besides  these  there  may  be  opposing  E.M.F's,  or  counter 
E.M.F's,  which  may  or  may  not  vary  with  the  amount  of  current 
in  the  circuit.  The  fall  of  potential  as  the  current  overcomes  resist- 
ance is  sometimes  called  an  E.M.F.,  as  it  is  measured  in  volts,  al- 
though it  is  more  common  to  call  it  "ohmic  drop"  or  simply  "drop." 


ELECTRIC   CIRCUITS.  79 

TABLE  SHOWING  THE  DIFFERENCE  BETWEEN  WIRE  GAGES. 


No. 

Brown  &  Sharpe 

Birmingham,  or 

Stubs 

New  British 
Standard 

Roebling,  or 
Washborn  & 
Moen. 

ooco 

.460 

•454 

•4 

•393 

000 

.40964 

-425 

.372 

.362 

00 

.36480 

.380 

•348 

•331 

o 

.32486 

•340 

.324 

•307 

I 

.28930 

.300 

.3 

.283 

2 

.25763 

.284 

.276 

.263 

3 

.22942 

.259 

.252 

.244 

4 

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Ooo7 

80  ELECTRIC  4L  CATECHISM. 

361.  What  are  the  distinctions  between  E.M.F.,  C.E.M.F.,  fall  of 
potential,  drop,  pressure,  difference  of  potential  and  ohmic  drop? 

E.M.F.  generally  refers  to  the  primary  cause  of  the  current,  or  the 
"electricity  moving  force."  Ohmic  drop,  or  simply  drop,  refers  to 
the  fall  of  potential  caused  by  the  resistance  met  by  the  current,  and 
equals  the  product  of  current  by  resistance ;  the  drop  may  refer  to 
the  resistance  of  the  entire  circuit,  or  to  that  in  any  specified  part  of 
it.  Counter  electromotive  force  is  usually  a  result  of  the  passage  of 
the  current  causing  some  inductive,  chemical,  or  equivalent  action, 
which  would  of  itself  tend  to  send  a  current  in  the  opposite  direction, 
a  simple  case  being  a  storage  battery  while  charging.  C. E.M.F.  is 
the  principal  factor  regulating  the  current  taken  by  a  motor  or  trans- 
former. C. E.M.F.  is  not  always  or  necessarily  simply  proportioned 
to  the  strength  of  the  current.  Fall  of  potential  is  a  general  term,  in- 
cluding drop  and  C. E.M.F.  Pressure  and  difference  of  potential  are 
general  terms  referring  to  any  of  the  others.  Voltage  is  a  gen- 
eral term  similar  to  difference  of  potential,  but  in  addition  calling 
attention  to  the  unit  used  in  its  measurement. 

362.  Can  each  of  these  quantities  be  measured? 

All  can  be  measured  directly  except  C. E.M.F.,  and  sometimes 
E.M.F.  An  electromotive  force  can  generally  be  measured  by  open- 
ing the  circuit  and  connecting  a  voltmeter  or  its  equivalent  to  the 
terminals  of  the  source  of  current.  When  this  can  not  be  done,  the 
total  E.M.F.  equals  the  voltage  at  the  terminals  plus  the  ohmic  drop 
through  the  generator.  Likewise,  C. E.M.F.  is  calculated  by  sub- 
tracting the  ohmic  drop  from  the  voltage  between  the  terminals. 

363.  Does  applying  a  certain  E.M.F.  to  a  conductor  having  one 
end  insulated  raise  the  whole  of  the  conductor  to  that  E.M.F.,  irre- 
spective of  its  length? 

When  no  current  is  flowing  through  a  conductor,  it  usually  has 
the  same  potential  throughout  its  length,  except  when  it  is  charged 
inductively.  For  example,  suppose  that  one  terminal  of  a  battery 
(say  the  negative)  giving  10  volts  is  connected  to  the  earth,  while 
the  other  terminal  (the  positive)  is  connected  with  one  point  of  an 
insulated  wire.  There  is  then  a  difference  of  potential  of  10  volts 
between  the  earth  and  the  positive  terminal  of  the  battery ;  the  same 
difference  of  potential  exists  betwen  the  earth  and  every  part  of  the 
ins.ulated  wire,  provided  it  is  not  acted  upon  by  any  E.M.F.  other 
than  that  of  the  battery.  If,  however,  the  insulated  wire  be  con- 
nected with  the  earth  either  directly  or  through  a  resistance,  current 
will  flow  through  the  wire,  causing  a  fall  of  potential  equal  to  product 


ELECTRIC  CIRCUITS.  81 

of  current  by  resistance ;  the  difference  of  potential  between  the  wire 
and  the  earth  then  falls  from  about  10  at  the  battery  terminal  toward 
zero  at  the  point  of  contact  with  the  earth.  When  a  conductor  is 
charged  inductively,  somewhat  as  discussed  in  Nos.  99  to  101,  the 
potential  may  vary  in  different  parts  when  no  current  flows.  For 
example,  suppose  that  one  end  of  a  conductor  is  near  a  body  charged 
negatively,  while  the  other  end  is  grounded ;  the  difference  of  poten- 
tial between  the  earth  and  the  conductor  then  varies  from  zero  at  the 
grounded  end  to  a  maximum  at  the  end  nearest  the  charged  body. 

364.  To  what  difference  of  potential  would  a  person  be  subjected 
if  he  should  touch  a  circuit  of  five  incandescent  lamps  connected  in 
series  between  the  trolley  wire  and  the  ground,  as  in  a  trolley  car? 

The  total  voltage,  or  difference  of  potential,  between  the  trolley 
and  the  ground,  which  is  usually  about  500  volts,  would  ordinarily 
be  divided  nearly  equally  among  the  different  lamps.  If  a  person 
should  stand  on  the  rail  or  on  wet  ground  and  should  touch  the  cir- 
cuit above  the  lamp  nearest  the  ground,  he  would  receive  a  shock 
due  to  approximately  100  volts;  touching  the  circuit  between  the 
second  and  third  lamps,  he  would  get  200  volts,  and  so  on.  It  would 
be  somewhat  risky  to  touch  the  circuit  at  higher  points.  The  severity 
of  a  shock  varies  with  the  voltage,  the  current,  the  part  of  the  body 
affected  and  the  general  state  of  the  person,  some  being  much  more 
sensitive  than  others.  It  is  believed  that  one-tenth  of  an  ampere 
through  a  vital  part  of  the  body  will  prove  fatal.  The  current  re- 
ceived will  obey  Ohm's  law  and  will  depend  on  the  resistance  of  the 
circuit.  The  resistance  of  the  human  body  varies  from  about  600 
ohms  in  the  case  of  criminals  fastened  in  the  electrocution  chair,  to  as 
high  as  80,000  ohms  between  the  fingers  of  two  hands.  The  re- 
sistance varies  with  different  people  and  with  the  cleanness  and 
moisture  of  the  skin.  The  current  received  will  also  depend  largely 
upon  the  resistance  between  the  person  and  the  ground ;  if  one  stands 
on  a  dry  wooden  platform,  the  resistance  to  ground  is  high  and  he 
can  touch  the  trolley  with  impunity ;  if  he  stands  on  the  rail  or  on 
wet  ground,  he  is  apt  to  get  a  severe  or  even  fatal  shock,  unless  he 
wears  insulated  shoes. 

365.  How  does  Ohm's  law  apply  to  a  circuit  containing  C.E.M.F.? 
The  C.E.M.F.  may  be  considered  as  reducing  the  total  E.M.F.,  and 

Ohm's  law  becomes:  Current  equals  E.M.F.  minus  C.E.M.F.  di- 
vided by  resistance,  or  (see  Nos.  1347  to  1373,  1405,  1420), 

E.M.F.  —  C.E.M.F. 
R 


82  ELECTRICAL  CATECHISM. 

366.  Give  an  example  showing  the  effect  of  C.E.M.F. 

A  75-hp  railway  motor  has  a  resistance  of  about  0.175  ohm.  If 
resistance  alone  determined  the  current,  it  would  take  2860  amperes 
on  a  5oo-volt  circuit.  Actually  the  current  varies  from  about  20  to 
about  200  amperes.  When  the  current  is  100,  the  ohmic  drop  is 
17.5  and  the  C.E.M.F.  is  the  difference  between  500  and  17.5,  that 
is,  482.5  volts. 

367.  How  can  the  drop  on  a  line  be  calculated? 

The  drop  equals  the  product  of  current  by  resistance,  and  there- 
fore equals  the  product  of  current  by  length  of  wire  by  resistance  per 
mil-foot  divided  by  the  square  of  the  diameter,  or  for  copper  wire, 

ii   X  L  X  I_n   X  L  X  I 

e  = T^ 

d  c.  m. 

When  the  outgoing  and  return  wire  are  of  equal  length,  the  distance 
between  the  supply  and  demand  is  used  instead  of  the  length  of  the 
wire,  the  distance  being  taken  as  one-half  the  length  of  the  wire. 
The  formula  then  becomes,  drop  equals  current  multiplied  by  dis- 
tance by  twice  the  resistance  per  mil-foot  divided  by  diameter 
squared  or  by  circular  mils,  or 

22  X  D  X  I        22  X  D  X  I 


e  = 


da  c.  m. 


368.     Give  an  example  shozving  the  calculation  of  drop  on  a  line. 

Suppose  current  of  100  amp.  is  carried  from  a  dynamo  to  a  motor 
200  ft.  away  by  a  copper  wire  having  a  diameter  of  0.325  in.  The 
drop  is 

2is  X  200  X   ioo        440,000 
e  =  -  —  =  --  7  —  =  4.2  volts. 

325  X  325  105,625 

369.  How  can  one  determine  the  size  wire  required  to  carry  a 
current  to  a  given  distance  with  a  given  loss? 

The  formula  in  No.  367  may  be  used,  being  transformed  for 
greater  convenience  into  the  form, 

_  22  X  D  X  I      22  X  D  X  I_ 


E  X  % 

or,  circular  mils  equal  22  times  distance  by  amperes,  divided  by  drop 
in  volts  or  in  per  cent,  of  voltage  delivered.  When  a  number  of 
calculations  are  to  be  made  for  the  same  drop,  as  in  wiring  a  build- 
ing, it  is  convenient  to  reduce  22/e  or  22/%E  into  a  single  factor  K. 
This  saving  of  labor  may  be  carried  further  by  using  wiring  tables. 


ELECTRIC  CIRCUITS.  83 

370.  Give  examples  showing  how  the  formula  is  used. 
Suppose  it  is  required  to  carry  10  amp.  80  ft.  with  a  drop  of  one 

volt.     The  circular  mils  required  in  the  conductor  are : 

22  X  80  X   10 
c.  m.  =   -  -  17600, 

which  is  a  little  larger  than  No.  8  B.  &  S.  G.  Since  No.  8  has  16510 
circ.  mils,  and  the  next  larger  number,  No.  7,  has  20820  circ.  mils, 
it  is  seen  that  No.  8  is  the  nearest  number,  and  this  would  be  used. 
Suppose  it  is  required  to  find  the  size  wire  necessary  to  carry  16 
amp.  120  ft.  with  a  loss  of  2  per  cent  on  a  no-volt  circuit.  The 
circular  mils  required  are: 

22  X   120  X   16     42240 
c.  m.= — — —  -     —=19200; 

IIO    X    O.O2  2.2 

this  is  almost  as  large  as  No.  7  B.  &  S.  G.,  which  would  be  used. 

371.  How  much  power  is  lost  in  a  line? 

The  power  lost  in  the  line  equals  the  product  of  current  by  ohmic 
drop,  or  the  product  of  current  by  current  by  line  resistance,  or  current 
squared  times  resistance  (see  Nos.  211,  213,  218,  361,  374).  To  re- 
duce the  loss  on  the  line,  while  delivering  a  certain  amount  of  power, 
it  is  necessary  to  reduce  either  the  resistance  or  the  current.  Reduc- 
ing the  resistance  involves  using  larger  wire,  which  costs  money ;  the 
current  may  be  reduced  by  raising  the  voltage  at  the  load,  for  the 
power  delivered  to  the  load  equals  the  product  of  current  by  voltage. 
For  example,  suppose  it  is  desired  to  deliver  48.4  kilowatts  over  a 
line  of  0.05  ohm  resistance.  At  no  volts  delivered,  we  have: 

Current  =  I  =  W/E  =  48400/1 10  =  440  amperes ; 

Line  drop  =  e  =  Ir  =  440  X  0.05  =  22  volts  =  20  per  cent. 

Line  power  loss  =  le  =  440  X  22  =  9680  watts  =  20  per  cent. 
If  the  load  could  be  arranged  for  220  volts  delivered,  we  have : 

Current  =  I  =W/E  —  48400/220  =  220  amperes. 

Line  drop  =  e  =  Ir  —  220  X  0.05  =  1 1  volts  =  5  per  cent. 

Line  power  loss  =  le  —  220  X  1 1  =  2420  watts  =  5  per  cent. 
It  is  thus  seen  that  doubling  the  voltage  for  delivering  a  given 
amount  of  power,  cuts  the  power  loss  and  the  percentage  line  drop 
to  one-fourth.  In  other  words,  the  line  loss  varies  inversely  as  the 
square  of  the  voltage  delivered.  Engineers  therefore  desire  to  use 
as  high  voltage  as  practicable.  For  distribution  circuits,  the  limit  is 
determined  by  considerations  of  vested  interests  and  safety  in  the 
receiving  apparatus  such  as  lamps  and  motors ;  for  transmission  cir- 
cuits, insulation  governs. 


84  ELECTRICAL  CATECHISM. 

372.  How  are  wiring  tables  made? 

Four  factors  are  combined  in  tabular  form :  ( i )  the  drop,  either 
in  volts  or  in  percentage  of  line  voltage;  (2)  the  current,  or  lamps 
(to  be  multiplied  by  current  per  lamp),  or  horsepower  (to  be  multi- 
plied by  current  per  horsepower  at  assumed  voltage)  5(3)  the  length 
of  wire  or  distance  between  supply  and  load;  (4)  the  size  of  con- 
ductor, in  diameter,  area  or  gage  number.  Assuming  any  one  of 
the  four  factors  constant,  such  as  drop,  two  series  of  values  are 
assumed  for  two  other  factors;  corresponding  values  are  then  cal- 
culated for  the  remaining  factor.  The  two  series  of  assumed  values 
are  the  headings  for  lines  and  columns.  In  calculating  or  using 
wiring  tables,  care  should  be  taken  not  to  exceed  the  current  carry- 
ing capacity  limits  of  the  insurance  rules.  (See  No.  418.) 

373.  Is  the  per  cent  drop  calculated  from  the  voltage  at  the 
dynamo,  or  that  at  the  lamps? 

It  is  commonly  calculated  from  the  volts  at  the  lamp,  but  ordinarily 
it  does  not  make  sufficient  difference  to  change  the  size  wire  a  single 
number,  so  that  either  voltage  may  ordinarily  be  taken  as  the  base. 

374.  Are  the  above  rules  for  wiring  correct  for  circuits  carrying 
alternating  currents? 

They  are  correct  for  distances  usually  met  in  interior  wiring.  For 
circuits  several  miles  long,  induction  effects  are  liable  to  cause  a 
greater  drop  than  is  indicated  by  the  simple  rules.  For  such  cases, 
the  calculations  are  more  complicated.  An  excellent  discussion  of  the 
subject,  with  convenient  tables  and  curves,  may  be  found  in  a  paper 
by  Mershon  in  American  Electrician,  June,  1897,  Vol.  IX.,  page  221. 
See  also  Section  1 1  of  Standard  Handbook  for  Electrical  Engineers. 

375.  Does  Ohm's  law  apply  to  circuits  carrying  alternating  cur- 
rents? 

Not  always  in  its  simple  form.  In  a  series  circuit,  the  sum  of  the 
separate  voltages,  or  drops,  may  be  considerably  greater  than  the 
total  E.M.F.  In  fact,  any  one  of  the  several  E.M.F's  may  be  greater 
than  the  whole.  Also  in  a  branched  circuit,  the  current  in  one  branch 
may  be  greater  than  the  whole.  With  a  given  E.  M.  F.  impressed  on  a 
circuit,  the  current  depends  not  simply  upon  the  resistance,  but  also 
upon  the  self-induction  and  capacity  of  the  circuit  and  upon  the  fre. 
quency.  This  subject  is  considered  in  Chapter  XII.  under  "Alter- 
nating Currents." 

376.  What  are  good  books  on  electrical  wiring  construction ? 

"  Electrical  Contracting,"  by  L.  J.  Auerbach ;  "  Electric  Light 
Wiring,"  by  C.  E.  Knox ;  "  Modern  Electrical  Construction,"  by 
Horstman  and  Tousley;  "Wiring  Handbook,"  by  C.  P.  Poole; 
"  Standard  Wiring,"  by  Cushing. 


CHAPTER  IV. 

ELECTRICITY  AND  HEAT. 

400.  What  is  the  relation  between  electricity  and  heat? 

Since  both  are  forms  of  energy,  one  may  be  changed  into  the  other. 
It  is  easy  to  change  electrical  energy  into  heat  energy  with  perfect 
efficiency,  so  that  the  heat  developed  is  equal  to  the  electrical 
energy  transformed.  It  is  not  so  easy  to  change  heat  energy  into 
electrical  energy,  the  processes  being  apparently  more  complicated 
and  being  less  efficient.  It  is  practicable  to  change  1000  watts  of 
electrical  energy  into  an  equal  amount  of  heat  energy,  but  1000  watts 
of  heat  energy  can  not  be  changed  into  1000  watts  of  electrical  energy 
by  any  process  yet  developed. 

401.  How  can  heat  energy  be  changed  into  electrical  energy? 
The  most  familiar  method  is  to  apply  the  heat  to  a  steam  boiler 

and  drive  an  engine  and  dynamo.  A  good  boiler  will  transfer  to 
the  water  and  steam  about  80  per  cent  of  the  energy  in  the  fuel ;  a 
good  steam  engine  will  change  from  10  to  20  per  cent  of  the  total 
energy  of  the  steam  into  mechanical  energy;  a  good  dynamo  will 
change  from  80  to  97  per  cent  of  the  mechanical  energy  received 
into  electrical  energy  available  for  use.  Putting  these  together  gives 
an  efficiency  of  from  6  to  15  per  cent  of  the  heat  energy  available  as 
electrical  energy,  the  rest  being  untransformed  heat  energy.  By 
the  use  of  gas  generators  and  gas  engines  driving  dynamos,  a  some- 
what higher  efficiency  may  be  obtained.  An  efficiency  of  nearly 
50  per  cent  is  claimed  for  certain  forms  of  gas  battery  in  which  the 
energy  in  the  gas  is  changed  into  electrical  energy  through  chemical 
action,  but  such  devices  have  not  been  reduced  to  a  commercial  basis. 
By  the  thermopile  a  theoretical  efficiency  of  about  12  per  cent  is  pos- 
sible, although  about  3  per  cent  is  the  highest  actually  obtained. 

402.  What  is  meant  by  a  thermo-couple? 

A  thermo-couple  is  an  electric  circuit  containing  two  conductors 
of  two  different  materials,  which  will  cause  a  thermo-electric  current 
when  their  two  junctions  are  at  different  temperatures.  A  common 
case  is  that  of  a  circuit  made  up  partly  of  iron  wire  and  partly  of 
copper  or  of  German  silver.  In  fact,  thermo-electromotive  forces 
are  set  up  in  any  circuit  in  which  there  is  any  lack  of  uniformity  of 


86  ELECTRICAL   CATECHISM. 

material  and  temperature.  A  circuit  made  up  partly  of  hard  cop- 
per wire  and  partly  of  the  same  wire  that  has  been  softened  by  an- 
nealing will  have  thermo-electric  currents  set  up  if  the  wire  is 
heated  at  the  point  where  the  hard  wire  joins  the  soft  wire.  The 
amount  of  the  current  is  governed,  of  course,  by  Ohm's  law,  and  the 
electromotive  force  depends  upon  the  difference  of  temperature  and 
also  upon  the  materials.  When  large  thermo-electric  currents  are 

t     Iron 
Hot 

FIG.  402.— ELEMENTARY  THERMOPILE. 

desired,  the  different  materials  should  be  chosen  with  great  care. 
Much  experimenting  has  been  done  to  discover  the  combinations  that 
give  the  best  results,  and  special  alloys  are  used  by  manufacturers 
of  thermo-piles.  Iron  and  German  silver  are  frequently  used  for 
experimental  work,  since  they  are  easily  obtained,  and  also  give 
comparatively  high  electromotive  force.  Thermo-electric  currents 
were  observed  by  Johann  Seebeck,  a  German  physicist,  in  1821. 

403.  Are  thermo-electric  currents  common? 

They  are  more  common  than  is  often  suspected.  They  often  make 
trouble  with  accurate  electrical  measurements,  since  it  is  frequently 
impossible  to  keep  all  parts  of  the  apparatus  at  the  same  tempera- 
ture; thermal  currents  are  thus  set  up,  which  interfere  with  exact 
measurements.  For  example,  in  measuring  resistance  with  the 
Wheatstone  bridge,  the  currents  heat  the  circuit  slightly,  and  this 
inequality  of  temperature  sets  up  thermal  currents,  which  affect  the 
galvanometer  after  the  main  current  has  been  cut  off,  so  that  it  is 
difficult  to  obtain  an  exact  balance.  Thermal  currents  are  always 
of  low  electromotive  force,  being  less  than  one-thousandth  of  a  volt 
per  junction,  so  that  they  are  troublesome  only  in  circuits  of  small 
resistance  and  with  delicate  instruments.  It  is  suspected  that  elec- 
trolytic troubles  are  sometimes  started  by  thermo-electric  currents, 
although  this  seems  not  to  have  been  verified. 

404.  What  is  a  thermopile? 

The  thermopile  is  an  apparatus  for  transforming  heat  energy  into 
electrical  energy.  It  is  the  thermo-couple  developed  by  making  a 
circuit  of  a  number  of  pairs  of  metals  or  alloys  so  arranged  that  al- 
ternate junctions  may  be  heated  while  the  remaining  junctions  are 
kept  at  a  lower  temperature,  somewhat  as  suggested  in  the  figure. 
If  the  two  conductors  are  iron  and  German  silver,  the  heat  causes 


HEATING   EFFECTS. 


8? 


current  to  flow  from  the  German  silver  to  the  iron  at  the  hot  junc- 
tions. Such  currents  are  called  ''thermo-electric."  The  heat  is  gen- 
erally supplied  by  one  or  more  gas  jets,  although  the  largest  sizes 
are  made  like  stoves  and  are  suitable  for  burning  coal.  The  outer 
junctions  are  usually  kept  comparatively  cool  by  means  of  extended 


FIG.  404A.— THERMOPILE. 


FIG.  404B.-COX  THERMO-ELECTRIC 
GENERATOR. 


plates,  sometimes  formed  into  tubes  so  as  to  assist  the  circulation  of 
cooling  air;  in  the  Cox  thermopile,  which  probably  represents  the 
highest  development,  the  outer  ends  are  cooled  by  the  circulation  of 
water  in  the  jacket.  A  cylindrical  Noe  thermopile  and  a  larger  Gul- 
cher  apparatus  are  also  shown  in  the  accompanying  figures. 


FIG.  404c.-  NOE  THERMOPILE. 


FIG.  404o.— GULCHER  THERMOPILE. 


405.     For  what  purposes  are  thermopiles  used? 

They  are  used  for  obtaining  currents  where  great  steadiness  is  re- 
quired, such  as  in  chemical  analysis.  They  are  suitable  for  all  pur- 
poses for  which  a  closed  circuit  battery  would  be  used,  that  is,  when 
currents  of  uniform  strength  are  desired  for  long  periods.  They 
are  suitable  for  electro-plating,  for  operating  telegraph  lines,  small 


88  ELECTRICAL   CATECHISM. 

motors,  fans,  or  for  other  purposes.  Current  from  a  thermopile  is 
generally  cheaper  and  less  troublesome  than  when  it  must  be  ob- 
tained from  a  primary  battery.  Thermopiles  are  also  used  for  meas- 
uring radiant  energy  and  for  measuring  temperatures. 

406.  Are  thermopiles  suitable  for  electric  lighting? 

They  have  not  been  constructed  in  sufficiently  large  sizes,  except 
for  small  lamps.  They  can  not  compete  with  engines  and  dynamos, 
both  because  of  difficulty  in  making  large  sizes,  and  because  of  the 
greater  efficiency  of  the  engine  and  dynamo. 

407.  How  is  a  thermopile  used  for  measuring  radiant  energy? 
Thermopiles  for  such  work  are  arranged  in  rectangular  form, 

somewhat  as  shown  in  the  figure.     When  one  end  is  exposed  to  any 
radiation,  such  as  the  sunlight  or  the  heat  radiated  from  a  fire  or 
lamp,  that  end  absorbs  heat  and  becomes  warmer  than  the  other  end, . 
the  result  being  a  thermo-electromotive  force.     If  the  wires  leading 


FIG.  407.— THERMOPILE  AND  GALVANOMETER. 

from  the  thermopile  are  attached  to  a  galvanometer,  the  electromotive 
force  and  current  can  be  measured  by  comparing  the  amount  of  the 
thermal  current  obtained  when  the  end  of  the  thermopile  is  directed 
successively  toward  different  sources  of  energy;  the  relative  inten- 
sities of  their  radiations  are  indicated  by  the  current  through  the 
galvanometer.  (See  Nos.  828  to  835.) 

408.     How  is  a  thermopile  used  for  measuring  temperature? 

Such  use  depends  upon  the  fact  that,  within  certain  limits,  the  cur- 
rent in  a  thermal  couple  varies  directly  with  the  difference  between 
the  temperatures  of  the  hot  and  cold  junctions.  After  the  apparatus 
has  been  properly  calibrated  by  keeping  one  junction  at  a  fixed  tem- 
perature, while  that  of  the  other  is  heated  to  different  known  tem- 
peratures, the  current  produced  being  carefully  noted  at  each  step, 
the  apparatus  can  be  used  to  determine  the  temperature  of  any  de- 
sired spot  by  placing  one  junction  at  that  spot  while  the  other  is  kept 


HEATING   EFFECTS.  89 

at  a  standard  or  known  temperature.  The  temperature  at  the  spot 
desired  can  then  be  calculated  from  the  current  through  the  thermo- 
electric circuit.  A  modification  is  sometimes  used  when  it  is  desired 
to  keep  two  points  at  equal  temperatures  :  one  junction  of  a  thermal- 
couple  is  placed  at  each  of  the  two  points,  and  if  no  current  flows 
through  the  circuit,  it  indicates  that  the  two  junctions  are  at  the 
same  temperature.  Zeleny  thus  detects  heating  of  grain  in  elevators. 

409.  Is  the  thermo-electric  effect  reversible? 

Peltier  discovered  in  1834  that  a  current  through  a  junction  of  two 
metals  would  heat  or  cool  it  according  to  the  direction  of  the  current, 
heating  being  observed  if  the  direction  of  the  current  was  the  same 
as  that  which  would  have  been  produced  by  cooling  the  junction,  and 
the  reverse.  This  "Peltier  effect"  acts  something  like  a  counter- 
electromotive  force  in  the  thermopile.  Various  attempts  have  been 
made  to  utilize  the  Peltier  effect  for  artificial  refrigeration,  but  such 
seem  not  to  be  commercially  successful. 

410.  Are  there  any  methods  for  obtaining  electricity  from  heat 
other  than  the  thermopile  and  the  engine  and  dynamo? 

There  are  quite  a  number  of  methods*  in  which  electricity  may  be 
obtained  from  heat  energy.  One  general  class  depends  upon  the 
maintenance  of  a  constant  difference  of  temperature  between  differ- 
ent parts  of  the  circuit.  Another  general  class  depends  upon  a 
changing  temperature  of  the  same  part.  The  latter  class  includes 
two  quite  distinct  methods  which  may  be  called  "pyro-electric"  and 
"pyro-magnetic." 

411.  What  are  pyro-electric  generators? 

Theophrastus  (about  450  B.  C.)  described  what  he  called  an  "elec- 
tric stone"  that  had  the  power  of  attracting  light  substances  when 
heated.  Aside  from  atmospheric  electricity,  this  seems  to  have  been 
about  the  first  knowledge  of  what  we  now  know  as  static  electricity. 
Certain  stones  and  crystals  become  charged  positively  at  one  end 
and  negatively  at  the  other  when  heated,  and  then  show  the  attractive 
and  repellant  effects  of  static  electricity.  The  tourmaline  takes  its 
name  from  the  Ceylonese  word  "tournamal,". meaning  "ash  col- 
lector." The  electrification  is  due  to  the  change  of  temperature. 
No  commercial  use  is  made  of  such  electrification,  except  possibly  to 
increase  the  sale  of  certain  stones. 

412.  What  are  pyro-magnetic  generators? 

These  are  devices  based  upon  a  discovery,  made  by  Gilbert  about 
1600  A.  D.,  that  iron  and  other  magnetic  substances  lose  their  mag- 


90  ELECTRICAL   CATECHISM. 

netic  properties  when  heated  white  hot.  In  1868,  Gore  made  use  of 
this  phenomenon  for  varying  the  magnetic  flux  through  a  coil  and  so 
inducing  a  current.  This  was  further  developed  by  Edison,  Menges 
and  Tesla,  but  none  of  them  arrived  at  commercial  success.  The 
same  phenomenon  has  been  applied  to  the  operation  of  small  motors 
by  Thomson  and  Houston,  McGee,  Schwedoff,  Cooper,  Berliner, 
Edison  and  Menges. 

413.  Has  heal  been  used  in  connection  with  chemical  action  for 
developing  electricity? 

This  has  been  tried  by  a  number  of  people.  In  1881,  Kendall  ob- 
tained currents  from  platinum  plates  heated  to  redness  in  hydrogen 
and  other  gases,  and  with  melted  salts  or  glass,  getting  about  one- 
half  volt  per  cell.  Following  some  experiments  of  Becquerel  in  1855, 
Jablochkoff  in  1877,  and  Brard  in  1882,  constructed  what  they  called 
"electro-generative  slabs,"  composed  of  carbon,  nitrate  of  potash 
and  copper,  which  produced  electricity  by  their  own  combustion 
when  thrown  into  a  fire.  In  1886,  Case  announced  a  kind  of  cell 
based  upon  the  fact  that  chromic  chloride  dissolves  metallic  tin 
when  heated  to  the  boiling  point,  and  precipitates  it  when  cooled; 
this  cell  developed  an  electromotive  force  of  0.26  volts.  In  1883, 
Acheson  developed  2.25  volts  by  heating  manganese  binoxide  be- 
tween two  concentric  cylinders.  In  1887,  Ettinghauser  and  Nernst 
obtained  an  E.M.F.  of  0.00125  volts  by  sending  heat  through  a  con- 
ductor in  a  strong  magnetic  field,  apparently  converting  some  of  the 
heat  into  electricity.  In  1886  to  1889,  Acheson  worked  along  this 
line  and  devised  a  number  of  plans  for  changing  electricity  into  heat 
in  the  influence  of  a  magnetic  field.  In  1896,  Jacques  brought  .out 
a  cell  in  which  current  was  obtained  by  heating  iron  and  carbon  elec- 
trodes with  caustic  soda. 

414.  Is  the  heating  effect  of  the  electric  current  of  much  practical 
importance? 

The  heating  effect  must  be  taken  into  account  in  the  design,  con- 
struction and  operation  of  almost  all  electrical  apparatus,  to  prevent 
excessive  rise  of  temperature.  On  the  other  hand,  the  heating  effect 
is  desired  for  many  purposes,  for  it  is  the  basis  of  many  useful  ap- 
plications of  electricity. 

415.  What  governs  the  amount  of  heat  from  a  current? 

It  varies  both  with  the  current  and  with  the  resistance.  No  sub- 
stance is  known  that  has  perfect  conductivity,  that  is,  every  conductor 
has  more  or  less  resistance.  (See  Nos.  338  to  354).  The  passage 
of  current  through  resistance  causes  a  certain  fall  of  potential  or 


HEATING   EFFECTS.  91 

voltage,  which,  by  Ohm's  law,  equals  the  product  of  current  by  re- 
sistance. The  product  of  current  by  this  drop  represents  the  power 
lost  or  changed  from  electricity  into  heat,  and  may  be  expressed  in  an 
equation, 

W  =  IXE  =  IXIXR  =  i'R. 

The  product  of  current  by  drop,  or  of  current  squared  by  resistance, 
when  multiplied  by  the  time  during  which  the  current  passes,  rep- 
resents the  amount  of  heat  developed.  The  unit  of  heat  may  be 
taken  as  the  joule,  which  equals  the  heat  developed  by  I  watt  acting 
for  one  second ;  or  it  may  be  taken  as  the  calorie,  which  equals  the 
amount  of  heat  necessary  to  raise  the  temperature  of  I  gram  of  water 
I  deg  C,  and  equals  4.2  joules,  or  4.2  volt-ampere-seconds,  or  4.2 
watt-seconds.  (See  Nos.  213,  220  and  221.)  The  number  of 
calories  may  thus  be  expressed  by  the  equation, 

watts  X  seconds 

Calories  ==-•-  -=  0.238  X  watts  X  seconds 

4.2 

=  0.238  X  current  X  drop  X  time 

=  0.238  X  current  X  current  X  resistance  X  time 

=  0.238  X  I  X  E  X  T 

=  0.238  X  I  X  I  X  R  X  T. 

416.  What  are  some  of  the  ways  in  which  the  heating  effect  may 
be  avoided  or  made  small ? 

Conductors  for  carrying  current  must  be  sufficiently  large,  or  there 
will  be  not  only  an  excessive  loss  on  the  line,  but  the  temperature 
of  the  conductors  will  be  liable  to  rise  so  high  as  to  endanger  the  in- 
sulation or  even  to  set  fire  to  surrounding  substances.  For  this  rea- 
son the  insurance  companies  and  the  electrical  interests  have  made  a 
careful  study  of  the  conditions  under  which  electricity  may  cause  fire 
risks,  and  have  adopted  sets  of  rules  governing  both  the  size  of  wire 
that  must  be  used  for  certain  currents  and  also  the  manner  of  plac- 
ing and  insulating  the  conductors.  Copies  of  the  "  National  Elec- 
trical Code  "  may  be  had  on  application  to  any  city  or  state  Board  of 
Underwriters. 

417.  How  is  the  safe  carrying  capacity  of  a  conductor  deter- 
mined? 

For  "wiring"  purposes,  that  is,  for  use  as  conductors  for  electric 
light  or  power,  the  tables  of  the  National  Electrical  Code  apply.  The 
size  of  wire  needed  in  other  situations  depends  so  largely  upon  cir- 
cumstances that  no  definite  rule  can  be  laid  down.  The  rule  in  the 
United  States  Navy  is  1000  amp.  per  square  inch,  equivalent  to  1273 
circ.  mils  per  ampere.  The  moving  parts  of  dynamos  and  motors 


92 


ELECTRICAL  CATECHISM. 


have  the  copper  wire  proportioned  for  about  350  to  700  circ.  mils  per 
ampere,  while  the  stationery  coils  have  850  to  1400  circ.  mils  per 
ampere.  The  time  in  use,  ventilation  and  allowable  rise  of  tem- 
perature vary  so  widely  that  no  definite  general  rule  can  be  laid  down. 

418.  What  is  the  rule  of  the  National  Electrical  Code  relating  to 
currents  and  sizes  of  wire? 

Conductors  must  be  made  of  practically  pure  copper,  or  if  of  other 
material,  must  be  made  enough  larger  to  secure  equal  conductivity. 
No  wire  must  carry  more  than  the  current  indicated  in  the  following 
table  (see  also  table  on  page  76)  : 

OF  CARRYING  CAPACITY  OF  COPPER  WIRES 


B.  &  S.  G. 

Circular 
Mils 

Weather- 
proof 
Wires. 
Amperes 

Rubber- 
Covered 
Wires. 
Amperes 

Circular  Mils 

Rubber- 
Covered 
Wires. 
Amperes 

Weather- 
proof 
Wires. 
Amperes 

18 

1,624 

5 

3 

200,000 

200 

300 

16 

2,  £Cj 

8 

6 

300,000 

270 

400 

14 

4,107 

16 

19 

400,000 

330 

500 

12 

6-530 

23 

17 

500,000 

39° 

5QO 

10 

10,380 

32 

24 

600,000 

45° 

680 

8 
6 

16,510 
26,250 

46 
65 

3 

700,000 

800,000 

500 
550 

760 
840 

5 
4 

33,loo 
41,740 

77 
92 

§ 

900,000 

1,000,000 

600 
650 

920 

,000 

3 

52,630 

no 

76 

1,100,000 

690 

,080 

2 

66,370 

131 

90 

1,200,000 

730 

,150 

I 
0 

83,690 
10^.500 

II! 

107 
127 

1,300,000 
1,400,000 

770 
810 

,220 
,290 

00 

133,100 

220 

150 

1,500,000 

850 

,360 

000 

167,800 

262 

177 

1,600,000 

890 

,43° 

0000 

211,600 

312 

210 

1,700,000 

930 

,490 

1,800,000 

970 

,550 

1,900,000 

1,010 

,610 

2,000,000 

1,050 

,670 

The  lower  limit  is  specified  for  rubber-covered  wires,  to  prevent 
decentering  of  the  wire  and  deterioration  of  the  insulation  by  the  heat 
but  not  from  fear  of  igniting  the  insulation.  The  question  of  drop  is 
not  taken  into  consideration  in  the  above  tables.  The  carrying  ca- 
pacity of  sixteen  and  eighteen  wire  is  given,  but  no  smaller  than  four- 
teen is  to  be  used,  except  in  pendants  and  in  fixtures.  The  choice  be- 
tween weatherproof  and  rubber-covered  wires  depends  upon  circum- 
stances. As  a  general  rule,  rubber-covered  wire  is  required  for  use 
inside  of  buildings. 


HEATING   EFFECTS, 


93 


419.  What  governs  the  rise  of  temperature  in  a  conductor  carry- 
ing a  current? 

Whenever  current  passes  through  a  conductor,  more  or  less  elec- 
trical energy  is  unavoidably  changed  into  heat  energy  (see  No.  415), 
and  the  temperature  rises  until  the  heat  lost  by  the  conductor  equals 
that  received  from  the  current. 

420.  What  becomes  of  the  heat  liberated  in  a  conductor? 

It  is  carried  away  by  conduction,  by  convection  and  by  radiation. 
Some  of  it  is  carried  away  by  conduction  along  the  wire  to  places 
where  the  temperature  may  be  lower.  Some  of  it  is  carried  away  by 
convection  currents  of  air.  Some  is  carried  away  by  radiation,  a 
process  similar  to  that  by  which  one  feels  the  direct  heat  from  a  fire 
or  from  the  sun.  The  amount  carried  off  by  convection  and  by 
radiation  depends  upon  the  difference  between  the  temperature  of  the 
conductor  and  that  of  the  surrounding  objects,  upon  the  character 
of  the  surface  of  the  conductor,  and  upon  the  freedom  of  circulation 
of  air. 

421.  How  can  excessive  heating  of  conductors  be  avoided? 

By  proper  design  in  the  first  place,  making  the  conductors  amply 
large  for  the  current  to  be  carried.  In  the  next  place,  there  should 
be  in  each  circuit  suitable  safety  devices,  such  as  fuses  or  circuit 
breakers,  that  will  open  the  circuit  when  the  current  exceeds  a  safe 
amount. 

422.  What  is  a  circuit  breaker? 

A  circuit  breaker  is  an  electromagnetic  device  for  opening  an  elec- 
tric circuit  when  the  current  varies  beyond  certain  desired  limits. 
Generally  it  is  designed  to  open  the  circuit  when  the  current  exceeds 
a  certain  amount,  although  for  some  purposes  it  is  arranged  to  open 


FIG.   422.— CIRCUIT  BREAKER. 


the  circuit  when  the  current  falls  below  a  fixed  amount.  It  is  operated 
by  an  electromagnet  whose  core  is  attracted  so  as  to  trip  a  release 


94  ELECTRICAL   CATECHISM. 

f 

trigger  or  detent,  and  allow  a  spring  to  throw  open  the  switch  when 
the  current  exceeds  the  limit  set.  Circuit  breakers  are  frequently 
used  instead  of,  or  to  supplement,  fuses  with  dynamos  and  motors. 

423.  What  is  a  fuse? 

There  are  two  entirely  different  devices  called  by  the  same  name 
and  based  upon  the  heating  effect  of  the  current,  one  being  used  for 
firing  explosives.  The  safety  fuse  referred  to  In  connection  with  in- 
surance rules  is  a  special  part  of  an  electric  circuit,  designed  to  carry 
the  ordinary  amount  of  current,  but  to  melt  and  open  the  circuit  in 
case  the  current  becomes  great  enough  to  heat  any  other  part  of  the 
circuit  beyond  a  safe  temperature.  The  fuse  is  made  of  such  ma- 
terial and  dimensions  that  its  resistance  is  considerably  higher  than 
an  equal  length  of  the  rest  of  the  circuit.  Since  the  heat  developed 
equals  the  product  of  the  resistance  by  the  square  of  the  current,  the 
fuse  is  continually  at  a  higher  temperature  than  other  parts  of  the 
circuit  (except,  of  course,  lamps  and  other  heating  devices)  ;  as  its 
melting  temperature  is  generally  comparatively  low,  an  increase  of 
current  above  that  for  which  the  circuit  is  designed  will  raise  the 
fuse  to  its  melting  temperature,  so  that  it  will  fall  away  and  open  the 
circuit. 

424.  What  other  names  are  given  to  fuses? 

They  are  called  cut-outs,  safety  catches,  safety  fuses,  safeties,  fuse 
plugs.  The  fusible  part  is  sometimes  called  a  fuse  link,  a  fuse  plug, 
a  cartridge,  a  protector  or  sometimes  a  fusible. 

425.  What  are  the  requirements  of  a  fuse? 

It  should  melt  at  a  comparatively  low  temperature,  so  as  not  to  en- 
danger setting  fire  to  surrounding  substances.  It  should  melt  quietly, 
so  as  not  to  throw  melted  particles  where  they  might  cause  damage. 


FIG.  425.— FUSE  LINKS. 

It  should  have  hard  terminals,  so  that  the  contact  between  it  and  the 
rest  of  the  circuit  shall  continue  firm.  It  should  have  a  definite  melt- 
ing point  that  will  not  change  with  prolonged  heating.  It  should  be 


HEATING  EFFECTS.  95 

long-  enough  so  that  an  arc  cannot  be  maintained  between  the  ter- 
minals after  the  fuse  is  melted.  It  should  be  mounted  upon  a  non- 
combustible,  non-absorptive  base.  See  Nat.  El.  Code  rules,  52,  53. 

426.     What  substances  are  used  for  fuses? 

The  most  common  is  an  alloy  of  tin  and  lead,  such  as  "half  and 
half"  solder.     Bismuth  is  frequently  added  to  the  alloy  to  lower  the 
melting  point.      Copper  wire  is  sometimes  used  in  exposed  places 
where  there  is  no  danger  from  flying  particles  or  from  the  high  tern 
perature  of  the  fuse  with  normal  current.     Iron  wire  should  not  be 


FIG.  426.— FUSE  WIRE, 

used,  since  it  burns  when  melted  and  is  very  liable  to  carry  fire. 
Numerous  "tested"  fuses  of  more  or  less  secret  alloys  are  upon  the 
market.  Aluminum  is  coming  into  extensive  use  as  a  fuse  metal 

427.     What  determines  the  current  required  to  melt  a  fuse? 

It  depends  upon  the  diameter,  more  current  being  required  to  melt 
a  thick  wire  than  a  thin  one ;  it  depends  upon  the  melting  tem- 
perature of  the  material,  some  alloys  melting  at  low  temperatures ;  it 
depends  upon  the  specific  resistance  of  the  fuse  metal,  also  upon  its 
temperature  coefficient  (that  is,  the  amount  the  resistance  changes 
with  temperature),  since  the  resistance  determines  the  amount  of 
heat  developed  by  the  passage  of  a  given  current ;  it  depends  upon 
the  cooling  effect  of  the  terminals,  and,  therefore  (within  certain 
limits),  upon  the  length  of  the  fuse  wire;  it  depends  upon  the  con- 
ditions for  dissipating  heat  by  convection  and  radiation.  The  quick- 
ness with  which  a  fuse  will  melt  after  the  current  has  reached  the 
limit  depends  upon  the  specific  heat  of  the  metal,  that  is,  upon  the 
number  of  heat  units  required  to  raise  the  temperature  I  deg.,  and 


96  ELECTRICAL   CATECHISM. 

also  depends  upon  the  latent  heat,  or  the  amount  of  heat  that  disap- 
pears in  melting  the  substance.  The  current  required  to  "blow"  a 
fuse  varies  to  some  extent  with  age,  as  the  fuse  generally  stands  at 
a  comparatively  high  temperature  whenever  the  normal  current  is 
passing,  and  hence  is  liable  to  oxidize  more  or  less;  molecular 
changes  also  occur  in  alloys  maintained  at  a  high  temperature,  so  that 
the  melting  temperature  is  liable  to  change. 

428.  How  are  fuses  rated? 

Fuses  are  sometimes  rated  according  to  the  number  of  amperes  to 
be  taken  normally  by  the  circuit  they  are  to  protect.  For  example, 
a  lo-amp.  fuse  is  supposed  to  protect  a  circuit  whose  regular  cur- 
rent should  not  exceed  10  amp.,  and  to  open  the  circuit  if  the  current 
rises  above  say  12  amp.  It  is  also  common  to  rate  fuses  according 
to  the  current  they  will  certainly  carry,  for  example,  a  lo-amp.  fuse 
wire  will  carry  10  amp.  without  melting,  but  how  much  more  it  will 
carry  is  not  specified,  and  it  may  carry  even  20  amp.  before  melting. 
The  rules  of  the  underwriters  require  that  each  fuse  be  stamped  with 
about  80  per  cent  of  the  maximum  current  it  can  carry  indefinitely, 
thus  allowing  about  25  per  cent  overload  before  the  fuse  melts. 

429.  Does  a  fuse  wire  always  melt  with  the  same  current? 

The  fusing  current  is  liable  to  vary  50  or  even  100  per  cent,  ac- 
cording to  circumstances.  The  temperature  of  the  surrounding  air 
or  other  substances  affects  the  melting  current  greatly,  for  the  melt- 
ing temperature  is  about  constant,  while  the  rate  at  which  heat  from 
the  fuse  will  be  transferred  to  the  surroundings  depends  upon  the  dif- 
ference of  temperance  between  them  and  the  fuse ;  consequently,  a 
fuse  in  a  warm  place  will  be  melted  by  a  smaller  current  than  a  similar 
fuse  in  a  cold  place.  For  a  similar  reason,  a  fuse  in  an  inclosed  place, 
where  there  is  little  chance  for  the  heat  to  be  dissipated,  will  melt  with 
a  smaller  current  than  the  same  in  an  open  place.  If  the  current  in- 
creases gradually  to  that  which  would  ordinarily  melt  the  fuse,  the 
high  temperature  makes  the  fuse  wire  oxidize  rapidly;  this  some- 
times makes  a  sort  of  tube  of  oxide  which  will  not  break  even  after 
the  fuse  wire  inside  has  melted,  and  so  the  fuse  carries  more  than  it- 
rated  current.  The  fuse  wire  is  generally  made  of  soft  material, 
such  as  lead  or  solder,  and  the  alternate  heating  and  cooling  causes 
the  fuse  metal  to  flatten  and  spread  under  the  fastenings,  so  that  the 
contact  becomes  poor ;  this  causes  an  increased  amount  of  heating, 
which  causes  the  fuse  to  melt  at  a  lower  current  than  intended.  To 
prevent  such  loosening,  it  is  advised  that  fuses  be  soldered  to  hard 
copper  terminals.  Open  fuses  are  so  unreliable  that  circuit  breakers 


HEATING  EFFECTS. 


97 


are  preferred'  for  large  currents,  and  inclosed  or  "  protected  "  fuses 
generally   called   "  cartridge   fuses "   are   preferred   in   most   cases. 

430.     How  much  current  is  required  to  melt  a  wire? 

When  the  wires  are  open  to  the  air  and  when  long  enough  so  that 
the  cooling  effect  of  the  terminals  does  not  extend  to  near  the  middle 
of  the  wires  (the  fuse  wires  being  from  2  ins.  to  12  ins.  in  length,  ac- 
cording to  their  diameters),  the  currents  required  for  melting  are  ap- 
proximately as  indicated  in  the  following  table  for  copper,  lead  and 
solder  wires : 

TABLE  OF  FUSING  CURRENTS. 


B.  &  S.  Gage 

No. 

Diameter 
in  Inches 

FUSING  CURRENT  IN  AMPERES 

Copper 

Lead 

y,  Lead  and 
K  Tin 

36 

.005 

4 

34 

.006 

5 

.... 

.... 

32 

.008 

8 

I 

I 

30 

.010 

II 

.... 

.... 

28 

.013 

14 

2 

2 

26 

.016 

20 

3 

.... 

24 

.020 

30 

4 

3 

22 

.025 

42 

7 

5 

20 

.032 

60 

10 

7 

19 

.036 

70 

ii 

9 

18 

.040 

83 

13 

ii 

17 

•045 

100 

?5 

13 

16 

.051 

120 

17 

15 

15 

•057 

I4O 

20 

18 

14 

.064 

166 

22 

20 

13 

.072 

200 

27 

24 

12 

.O8l 

235 

30 

II 

.091 

280 

38 

35 

10 

.101 

335 

44 

4i 

9 

.114 

390 

5i 

48 

8 

.129 

450 

62 

58 

7 

.144 

520 

77 

73 

The  above  figures  are  calculated  from  a  law  discovered  by  Preece, 
that  the  diameter  (in  inches)  of  a  copper  wire  equals  the  cube  root 
of  the  square  of  the  quotient  of  amperes  divided  by  10,244.  For 
lead,  the  divisor  is  1379.  For  the  half  and  half  solder,  lead  and  tin, 
the  divisor  is  1318.  The  figures  for  lead  and  for  solder  agree  closely 
with  results  of  experiments  by  Bathurst.  Part  of  the  figures  for 
copper  fuses  have  been  tested  experimentally  and  found  closely 
correct. 


98  ELECTRICAL   CATECHISM. 

431.  Will  two  fuses  in  multiple  carry  twice  as  much  current  as 
one? 

They  will  if  each  carries  the  same  current,  and  if  the  two  are  so 
placed  that  neither  affects  the  heating  of  the  other.  Two  fuses 
twisted  together  or  placed  close  together  will  carry  less  than  twice 
the  current  of  one,  since  the  heat  from  the  two  is  not  carried  off  so 
easily  as  from  one.  If  one  fuse  has  more  resistance  than  the  other, 
as  might  occur  when  two  fuses  of  the  same  stock  but  of  different 
lengths  are  coupled  in  multiple,  the  one  having  the  smaller  resistance 
will  carry  more  than  its  half  of  the  current,  and,  consequently,  will 
melt  before  the  total  current  is  twice  the  melting  current  for  one  fuse. 

432.  How  can  a  multiple  fuse  be  adjusted  to  carry  any  desired 
current? 

If  the  fuses  are  placed  so  that  the  heat  from  one  does  not  affect  the 
other,  as  when  the  two  are  placed  in  separate  blocks,  their  joint  carry- 
ing capacity  may  be  adjusted  approximately  by  the  following  method : 
Suppose  it  is  desired  to  make  a  fuse  for  350  amp.  from  a  No.  12  bare 
copper  wire  which  fuses  at  about  235  amp.  Use  two  fuses  of  such 
length  (that  is,  of  such  relative  resistance),  that  when  one  carries  253 
amp.,  the  other  will  carry  the  difference  between  235  and  350,  the 
total  current,  or  115  amp.  This  may  be  done  by  making  one  fuse, 
say  2.35  ins.  long,  and  the  other  1.15  in.  The  shorter  one  will  then 

carry  -   -  of  the  total  current,  while  the  longer  one  carries  -  -  .     It 

would  be  better  to  double  these  lengths  so  that  the  flash  at  melting 
can  not  maintain  an  arc  across  between  the  terminals. 

433.  What  is  a  fuse  block? 

A  fuse  block  is  a  device  for  holding  a  fuse.  It  usually  consists  of 
brass  clamps  mounted  on  porcelain.  Generally  a  cover  is  provided, 
both  for  the  purpose  of  preventing  the  fuse  from  flying  when  melt- 
ing, or  "blowing,"  and  also  to  improve  its  looks,  and  to  reduce  the 
amount  of  "live"  surface  exposed  to  accidental  contact.  Fuse  blocks 
are  made  in  many  different  styles  and  for  many  different  situations* 
For  use  on  high  potential  circuits,  such  as  looo-volt  or  higher  pres- 
sure circuits  (and  often  for  loo-volt  circuits),  the  fuse  is  commonly 
placed  on  a  separate  cover  piece,  so  that  the  fuse  wire  can  be  clamped 
in  place  on  the  cover  before  being  connected  with  the  live  wires. 
(See  also  No.  437.) 

434.  What  are  the  principal  kinds  of  fuse  blocks? 

There  is  a  great  variety  to  suit  various  purposes.  For  use  in  elec- 
tric light  and  power  circuits,  fuse  blocks  or  "cut-outs"  may  be  class\° 


HEATING   EFFECTS.  99 

fied  into  "main  line  cut-outs"  and  "branch  blocks."  A  branch  block 
is  used  for  connecting  branches  to  a  larger  circuit,  the  fuses  being 
so  arranged  that,  if  trouble  occurs  on  the  branch,  it  is  cut  off  without 


FIG.  434A.— MAIN  LINE  CUT-OUTS. 


FIG.  434B.— BRANCH  BLOCKS. 


FIG.  434C.-FUSE  BLOCK  FOR  TRANSFORMER. 

opening  the  main  line.     As  its  name  indicates,  a  main  line  cut-out  is 
placed  in  the  main  line.     The  accompanying  figures  show  common 


100 


ELECTRICAL   CATECHISM, 


forms  of  main  and  branch  blocks  for  use  on  circuits  for  electric  light 
or  power  inside  buildings.  Fuse  blocks  for  use  out  of  doors  are  made 
weatherproof,  being  encased  in  cast-iron  boxes,  somewhat  as  shown 
in  the  accompanying  illustration  of  inclosed  fuse  for  transformers. 
For  circuits  using  very  small  currents,  such  as  telephones,  special 
forms  of  fuse  are  employed. 

435.     What  is  an  inclosed  fuse? 

In  an  inclosed  fuse  the  fusible  wire  is  placed  in  a  tube  or  in  a  cavity 
within  porcelain  or  some  other  insulating  substance.  The  space 
around  the  fuse  wire  is  sometimes  filled  with  some  substance  that  will 
prevent  the  formation  of  an  arc  after  the  melting  of  the  fuse.  One 


FIG.  435A.— INCLOSED  FUSE. 


FIG.  435B.— INCLOSED  FUSE. 


style  shown  in  the  accompanying  figure  has  the  fuse  wire  surrounded 
by  a  chemical  substance,  such  as  borax  or  sal  ammoniac,  that  unites 
with  the  lead  fuse  wire,  and  by  forming  a  non-conductive  substance, 


FIG.  435c.— INCLOSED  FUSE  WITH  INDICATOR. 

destroys  the  conductivity  of  the  fuse  in  case  its  temperature  rises 
above  a  specified  degree.     Sometimes  a  fine  fuse  is  placed  outside  the 


HEATING    EFFECTS. 


101 


tube  so  as  to  melt  after  tlje  main  fuse  inside,  and  thus  indicate 
whether  it  has  melted.  In  others,  such  as  the  Edison  fuse  plug,  the 
fuse  wire  is  encased  in  a  glass  or  porcelain  cup,  from  which  air 
drafts  are  excluded  by  the  cover.  Inclosed  fuses  are  more  re- 


FIG.  435D.— FUSE  PLUGS,  125  AND  250  VOLTS. 

liable  than  the  older  open  fuses,  because  the  fuse  wire  is  under 
practically  the  same  conditions  at  all  times,  and  will,  therefore,  melt 
at  a  more  definite  temperature.  They  are  now  generally  required. 

436.     What  is  a  detachable  fuse? 

The  fuse  terminals  of  a  detachable  fuse  block  are  attached  to  the 
cover  piece,  that  is,  to  a  piece  which  may  be  removed  from  the  station- 
ary block,  so  that  the  fusible  portion  may  be  placed  in  the  cover 


FIG.  436.— DETACHABLE  FUSE  BLOCK. 

or  "pocket"  while  entirely  disconnected  from  the  circuit,  the  con- 
nections being  made  when  the  cover  is  placed  in  position  on  the  base. 


102 


ELECTRICAL   CATECHISM. 


437.  What  is  a  protected  fuse? 

Some  manufacturers  make  fuse  blocks  in  which  the  fuse  is  en- 
closed in  a  porcelain  tube  or  channel,  having  a  small  aperture  through 
which  the  vapors  following  the  melting  fuse  can  escape,  the  arc  being 
blown  out.  The  tube  protects  the  fuse  from  air  drafts  and  from 
contact  with  other  substances,  and  also  fixes  it  at  a  definite  length. 
In  the  protected  fuses  the  fuse  wire  is  generally  in  a  piece  separate 
from  the  rest  of  the  block,  so  that  the  fuse  may  be  renewed  while  the 
cover  or  plug  piece  is  entirely  detached  from  the  circuit,  thus  making 
it  safer  to  handle  than  the  ordinary  open  fuse  block. 

438.  What  is  a  "bug"  cut-out? 

Bug  cut-outs  are  small  single  pole  cut-outs  for  placing  in  cramped 
and  awkward  places,  such  as  in  fixtures,  where  a  regular  fuse  block  is 
impracticable.  Their  use  is  avoided  whenever  possible. 


FIG.  438.— BUG  CUT-OUT. 

439.  What  is  the  difference  between  single-pole  and  double-pole 
cut-outs? 

Cut-outs  are  generally  made  double-pole,  that  is,  so  that  there  is  a 
fuse  on  each  side  of  the  circuit  to  give  protection  against  heavy  cur- 
rents, whether  caused  by  a  "short-circuit"  on  the  line  itself,  or  caused 
by  two  ground  connections  in  different  parts  of  the  circuit.  In  the 
case  of  large  currents  at  high  potentials,  it  is  common  to  use  two 


FIG.  439A.— THREE-WIRE  MAIN 
CUT-OUT. 


FIG.  439B.— THREE-WIRE   BRANCH 
BLOCK. 


single-pole  cut-outs,  in  order  to  maintain  a  safe  distance  between  the 
two  sides  of  the  circuit.  Three-wire  systems  require  a  fuse  in  each 
01  the  three  wires  at  each  branch. 


HEATING  EFFECTS. 


103 


440.     What  are  panel  boards  or  panel  cutouts? 

In  wiring  buildings  it  is  common  to  use  the  "  cabinet  system,"  in 


FIG.  440A.— DISTRIBUTION   PANEL,  CARTRIDGE   FUSES. 

which  the  lamp  circuits  radiate  from  one  or  more  distribution  centers 
where  the   fuses    (and   sometimes  the   switches)    are  collected   for 


L___      r  r  r 

FIG.  440B.-CABINET   PANELS,   PLUG  FUSES. 

greater  safety  and  convenience.    When  the  fittings  are  mounted  di- 
rectly on  a  slate  or  marble  slab,  it  is  called  a  panel  board,  fuse 


104 


ELECTRICAL  CATECHISM. 


panel  or  tablet ;  when  they  are  mounted  on  separate  porcelain  bases 
which  are  then  mounted  on  asbestos-covered  wood  or  on  iron,  it  is 
called  a  panel  cutout.  The  tablets  should  be  enclosed  in  cabinets  of 
non-combustible,  non-absorptive  material  such  as  slate,  marble,  steel, 
or  wood  protected  by  asbestos  or  slate. 

441.     What  kind  of  fuse  is  used  on  telephone  and  similar  lines? . 

The  currents  ordinarily  used  on  such  lines  are  so  small  that  foreign 
currents  of  less  than  one  ampere  may  cause  damage.  The  fuses 


FIG.  441A.— TELEPHONE  FUSES. 

must  therefore  be  much  more  sensitive  than  those  used  on  electric 
light  circuits.  A  common  form  is  a  piece  of  fine  fuse  wire  mounted 
on  a  strip  of  mica  with  copper  terminals.  The  "  grasshopper  "  fuse 
has  some  fine  high-resistance  wire  doubled  up  inside  a  plug  of  wax 
and  connected  by  a  bell-shaped  shell  and  a  stiff-wire  hook  to  a  fork 
and  a  spring;  an  excessive  current  will  soften  the  wax,  when  the 
spring  breaks  the  fine  wire.  The  McBerty  "  thermal  arrester  "  used 
by  the  Bell  companies  contains  a  rubber  cylinder,  B,  in  which  are 
a  number  of  turns  of  fine  high-resistance  wire  in  which  a  stray  cur- 
rent will  generate  so  much  heat  as  to  melt  the  special  solder  and 


FIG.   441  B.— THERMAL  ARRESTERS. 

allow  a  spring,  G,  to  push  the  central  pin,  P,  against  a  smaller 
spring,  F,  which  connects  the  line  with  the  earth ;  A  is  a  lightning 
arrester  with  a  couple  of  carbon  pieces  held  apart  by  a  strip  of  mica 
(see  No.  156)  ;  sometimes  a  plug  of  fusible  metal  is  placed  in  one 
carbon  so  as  to  melt  and  make  a  firm  connection  in  case  the  lightning 
discharge  is  accompanied  by  considerable  current ;  sometimes  auxili- 
ary strips  are  added  so  as  to  ring  a  bell  whenever  a  fuse  goes  out. 


HEATING  EFFECTS. 


105 


442.     What  is  meant  by  a  telephone  protector? 

A  telephone  protector  is  a  combined  fuse  and  lightning  arrester. 
The  arrester  usually  consists  of  pieces  of  flat  carbon  separated  by 
thin  pieces  of  mica  notched  or  perforated  so  as  to  leave  thin  air 


FIG.   442.— TELEPHONE   PROTECTORS. 


spaces  between  the  carbon  blocks.  The  fuse  may  be  of  a  type  to 
melt  and  open  the  circuit,  or  it  may  simply  connect  the  line  to  the 
earth,  as  explained  in  No.  441. 

443.     Are  fuse  wires  adaptable  for  fire  alarms? 

Fusible  metal  has  been  applied  in  several  ways  to  the  extinguishing 
of  fires  by  opening  sprinklers  when  the  temperature  exceeds  a  definite 
degree.  The  same  idea  has  been  applied  in  the  "montauk"  cable 


FIG.  443.— FIRE   DETECTOR   CABLE. 

for  giving  an  alarm  under  similar  circumstances.  The  cable  con- 
sists of  a  number  of  conductors,  the  inner  one  of  which  is  surrounded 
by  an  alloy  that  fuses  at  about  370  deg.  F.  If  the  cable  is  exposed 
to  a  high  temperature,  such  as  might  be  caused  by  an  incipient  fire, 
the  fusible  alloy  melts  and  completes  the  circuit  between  the  central 
wire  and  those  near  it,  thus  sending  in  an  alarm  through  an  electric 
bell  or  similar  device. 

444.  For  what  useful  purposes  is  the  heating  effect  of  the  current 
employed? 

The  heating  effect  is  used  in  safety  devices  such  as  the  fuses  noted 
above.  (See  Nos.  423  to  443.)  Moderate  degrees  of  temperature 
are  used  in  heaters  of  various  sorts,  such  as  stoves,  heating  pads,  tea 
kettles,  coffee  pots,  ovens,  smoothing  irons,  curling  iron  heaters, 


106 


ELECTRICAL   CATECHISM. 


soldering  coppers,  solder  or  babbitt  melting  pots,  glue  pots,  branding 
irons,  stamp  cancellers,  surgeons'  cauteries  and  the  like.  Higher 
temperatures  are  used  in  the  tempering  of  steel,  in  blasting  fuses,  in 
incandescent  lamps,  arc  lamps,  electric  welding  and  forging,  and  in 
various  electro-metallurgical  processes.  Applications  of  the  heat- 
ing effect  of  the  current  are  developing  continually.  The  heating 
effect  of  the  current  is  used  to  a  small  extent  in  electrical  measure- 
ments. Heat  is  developed  incidentally  in  connection  with  rheostats 
for  regulating  or  absorbing  electrical  energy. 

445.     What  is  a  blasting  fuse? 

A  blasting  fuse  is  illustrated  in  the  accompanying  figure,  in  which 
a  small  platinum  wire,  E,  may  be  heated  by  a  current  and  so  ignite 
an  explosive  mixture,  B,  which  will  ignite  a  still  larger  charge  out- 
side the  fuse.  Current  enters  and  leaves  through  the  wires,  C,  which 


FIG.  445.-BLASTING  FUSE  AND  MAGNETOS. 

are  long  enough  to  extend  beyond  the  surface  of  the  hole.  As  many 
as  forty  of  these  fuses  may  be  connected  in  series  and  fired  simul- 
taneously. Current  is  usually  furnished  by  a  small  magneto  dynamo 
similar  to,  but  more  powerful  than,  those  used  for  ringing  telephone 
bells. 

446.     What  is  a  rheostat  f 

A  rheostat  is  a  conductor  arranged  to  dissipate  electrical  energy 
into  heat.  It  may  be  used  simply  to  regulate  the  resistance  of  the 
circuit  of  which  it  is  a  part,  it  may  be  to  regulate  the  current  by 
changing  the  resistance  of  a  circuit,  it  may  be  to  dissipate  energy  as 
in  a  test,  it  may  be  to  develop  desirable  heat  from  electricity.  Usually 
the  rheostat  is  so  arranged  that  its  resistance  may  be  changed  at  will. 


HEATING    EFFECTS.  107 

447.  For  what  purposes  are  rheostats  used? 

Rheostats  are  connected  into  the  field  magnetizing  circuits  of 
dynamos  to  regulate  the  electromotive  force  of  the  dynamo;  they 
are  used  in  series  with  motors  to  make  them  start  slowly ;  they  are 
sometimes  used  in  the  field  circuits  of  motors  to  regulate  the  speed  f 
they  are  sometimes  used  to  absorb  the  power  of  a  dynamo  while  being 
tested  for  capacity  and  efficiency ;  they  are  used  to  regulate  the  cur- 
rent in  a  great  variety  of  apparatus  supplied  from  circuits  of  constant 
potential.  Rheostats  are  sometimes  used  to  vary  the  difference  of 
potential  between  the  terminals  of  apparatus  connected  to  constant 
potential  circuits. 

448.  Of  what  material  are  the  conductors  in  rheostats  composed? 
Metallic  wire  is  most  common,  although  carbon  and  graphite  are 

used  sometimes ;  it  is  not  unusual  to  employ  water,  made  conducting 
by  the  addition  of  salt  or  acid.  Iron  is  cheap  and  will  stand  a  high 
temperature.  Copper  has  high  conductivity,  and  is  convenient 
where  large  currents  at  low  voltage  must  be  controlled.  German  sil- 
ver has  high  resistance  and  low  temperature  coefficient,  and  is  used 
where  the  resistance  must  be  constant.  Other  alloys  are  used. 

449.  How  are  the  conductors  supported  in  rheostats? 

Where  the  energy  is  small,  as  in  telegraph  systems  and  in  standard 
resistances  in  Wheatstone  bridges  and  similar  laboratory  apparatus, 
the  wire  may  be  wound  in  coils  directly  upon  wooden  spools.  Where 
there  is  liable  to  be  considerable  heat,  the  conductors  must  be  sup- 
ported on  non-combustible  material,  as  is  required  by  the  insurance 
rules  covering  the  use  of  electricity  for  lighting  and  power  purposes. 
The  conductors  may  dissipate  their  heat  into  the  open  air  directly  or 
through  another  surrounding  medium,  such  as  enamel,  which  carries 


FIG.  449A.— ELECTRIC  AIR  HEATER. 


the  heat  to  cast-iron  base  plates.  The  conductors  may  be  wound  on 
slate  slabs,  on  a  framework  of  iron  rods  insulated  with  asbestos,  or 
on  insulated  metallic  spools  with  layers  of  asbestos  between  the  layers 
of  wire ;  the  wire  may  be  wound  into  coils  which  are  stretched  be- 


108 


ELECTRICAL   CATECHISM. 


tween  insulators  on  a  frame  of  iron  or  of  insulating  material.  When 
the  rheostat  is  to  be  used  for  only  a  short  time,  the  wire  is  sometimes 
wound  on  spools,  as  such  method  allows  a  large  amount  of  wire  in 
small  space,  and  the  capacity  of  such  a  rheostat  depends  .more  on  its 
capacity  for  taking  up  heat  than  on  radiation  and  convection.  When 
spiral  coils  are  employed,  provision  is  made  for  free  circulation  of 


->?l~ 


FIG.  449B.— SECTION  OF  ENAMEL  RHEOSTAT. 

air.  Sometimes  asbestos  tubes  are  placed  inside  coils  of  small  wire 
for  the  purpose  of  keeping  them  stiff,  while  the  convolutions  are 
close  together.  In  some  cases  the  resistance  wire  in  small  rheostats 
is  wound  on  flat  "cards"  of  mica  or  fuller  board,  which  are  then  bent 
and  packed  closely  together.  Absorption  rheostats  are  sometimes 
immersed  in  water  to  facilitate  cooling  and  thereby  vo  increase  their 
current  carrying  capacity.  Cast-iron  grids  stand  hard  service. 

450.     How  is  the  resistance  of  a  rheostat  varied? 

The  conductors  are  generally  all  in  series,  and  connections  are 
made  at  various  points  with  contact  blocks  upon  which  a  moving  arm 
sweeps,  so  as  to  include  more  or  less  of  the  resistance,  as  suggested 
in  the  figure.  Sometimes  the  resistance  is  reduced  while  the  carrying 


FIG.  450.— ADJUSTABLE  RHEOSTATS. 


capacity  is  increased  by  putting  more  or  less  resistance  units  in  mul- 
tiple, as  when  a  bank  of  incandescent  lamps  is  used  for  regulating 
a  current.  Combinations  of  the  two  are  made  occasionally,  the  high- 
est resistance  being  given  by  connecting  the  units  in  series,  while  the 


HEATING    EFFECTS. 


109 


resistance  is  reduced  by  cutting  out  one  section  after  another,  the  re- 
sistance being  still  further  reduced  by  putting  the  units  in  multiple. 

451.  How  is  a  water  rheostat  constructed? 

The  water  may  be  contained  in  almost  anything  convenient,  al- 
though it  is  generally  safer  to  use  a  vessel  of  insulating  material,  such 
as  a  wooden  box  or  barrel.  The  electrolyte  may  be  ordinary  water 
when  pressures  of  500  volts  or  higher  are  to  be  used.  It  is  cus- 
tomary, however,  to  improve  the  conductivity  of  the  water  by  the  ad- 
dition of  a  little  salt  or  washing  soda,  or  a  little  acid  of  some  kind. 
A  tablespoonful  of  salt  is  about  enough  for  a  barrel  of  water  when 
the  voltage  is  550.  More  salt  should  be  used  for  lower  voltages. 
The  conductor  terminals  are  usually  plates  of  iron  although  almost 
anything  will  do.  The  resistance  is  regulated  by  -langing  the  dis- 
tance between  the  plates,  or  the  amount  of  surface  of  the  plates,  or 
by  changing  the  density  of  the  solution,  the  latter  not  being  easily 
changed.  Rectangular  boxes  of  wood  or  stone  are  often  used. 

452.  How  arc  the  plates  of  a, water  rheostat  made? 

Two  pieces  of  iron  pipe  may  be  held  apart  by  two  boards  and 
let  down  into  the  barrel ;  or  one  may  be  fastened  to  the  side  of  the 
barrel  and  the  other  be  adjustable  up  and  down.  The  plates  may 


Am.Elec. 
FIGS.  452A,  452B  AND  452c.— WATER  RHEOSTATS. 

be  two  tubes  or  pieces  of  sheet  iron  held  apart  by  wooden  strips  and 
adjustable  up  and  down  by  ropes  over  pulleys,  as  suggested  in  Fig. 
a.  When  a  close  adjustment  for  small  currents  is  desired,  the  plates 


HO  ELECTRICAL  CATECHISM. 

may  be  cut  off  spirally,  as  suggested  in  Fig.  b.  A  convenient  plan 
is  to  bolt  a  plate  to  the  bottom  of  the  barrel,  being  careful  to  make 
a  tight  fit  and  to  paint  the  lower  side  well  with  pitch  or  asphalt ;  the 
upper  plate  may  be  a  hitching  weight  to  which  is  screwed  a  plate 
with  a  larger  surface,  the  whole  being  suspended  by  the  conductor, 
as  suggested  in  Fig.  c. 

453.  How  much  energy  may  be  dissipated  in  a  water  rheostat? 
A  barrel  containing  fifty  to  sixty  gallons  will  dissipate  an  equal 

number  of  kilowatts.  Smaller  rheostats  sometimes  take  care  of  20 
to  150  watts  per  cubic  inch  of  liquid.  The  allowable  current  varies 
from  .25  to  3  amp.  per  square  inch  of  surface  on  the  exposed  face  of 
one  electrode. 

454.  What  precautions  are  desirable  in  handling  water  rheostats? 

The  water  should  not  contain  too  much  salt  at  first,  since  the  re- 
sistance decreases  as  the  temperature  rises,  and  too  much  current  is 
liable  to  be  the  result.  The  salt  should  be  added  from  a  solution, 
since  it  takes  some  time  to  dissolve  and  diffuse  through  the  whole 
body  of  water;  hence  the  resistance  would  continue  to  change  for 
some  time  after  the  salt  was  added.  The  resistance  is  most  steady 
when  the  water  is  boiling  quietly;  it  takes  quite  a  time  before  this 
point  is  reached,  but  the  time  taken  to  warm  up  the  rheostat  is  useful 
in  getting  everything  into  working  order  before  the  actual  test  be- 
gins. The  water  rheostat  should  usually  be  kept  out  of  doors,  unless 
it  is  to  be  used  only  for  a  short  time,  as  the  fumes  are  corrosive. 
Great  care  should  be  taken  to  prevent  the  plates  from  actual  con- 
tact with  each  other,  which  might  cause  a  short-circuit  and  thus  in- 
jure the  dynamo.  In  many  cases  it  is  also  desirable  not  to  ground 
the  circuit,  as  may  easily  occur  with  leaky  barrels.  There  is  always 
more  or  less  risk  of  getting  shocks  in  adjusting  the  water  rheostat. 
It  is  an  excellent  plan  to  place  the  barrel  upon  timbers,  to  insulate  it 
from  the  ground.  When  the  water  is  kept  boiling,  it  is  desirable  to 
replenish  it  more  or  less  continuously,  so  as  to  keep  the  temperature 
and  resistance  constant;  for  this  purpose  a  hose  may  be  attached 
to  the  water  mains  and  the  valve  adjusted  to  give  the  right  amount 
of  flow;  care  must  be  taken  against  using  wire  wound  hose,  as  this 
would  ground  the  circuit  and  might  shock  the  attendant. 

455.  How  are  electric  heating  and  cooking  utensils  made? 
These  generally  have  conductors  of  iron  or  of  high  resistance 

alloy  wound  upon  asbestos  or  mica  or  imbedded  in  enamel  on  cast 
iron.  The  heat  from  the  wire  travels  through  the  mica,  asbestos  or 


HEATING  EFFECTS. 


Ill 


enamel  to  the  surrounding  case,  which  may  be  a  soldering  copper, 
a  flat  iron,  a  cooking  utensil  or  any  other  desired  receiver. 


FIG.  455.-CURLING  IRON  HEATER. 


FIG.  455.— AIR  HEATER. 


456.     How  much  energy  is  required  by  electric  heating  devices? 

Referring  to  no-volt  circuits,  a  6J-lb.  sad  iron  for  common  house- 
hold use  requires  4  amp.,  or  for  laundry  use,  where  the  work  is 
rushed,  5  or  6  amp. ;  a  polishing  iron  requires  2.5  amp.,  and  an  i8-lb. 


FIG.  456A.-DISC  HEATER. 


FIG.   456B.-LAUNDRY  IRON. 


FIG.  456C.-CHAFING  DISH. 


FIG.  456D.— BOTTLE   WARMER. 


goose  iron,  5  amp. ;  a  4|-in.-plate  stove  requires  1.9  amp.,  which  cur- 
rent will  make  it  hot  in  about  two  minutes.  A  larger  disc  heater, 
6  ins.  in  diameter,  requires  5.5  amp. ;  a  single  griddle  requires  the 
same  current,  and  a  three-section  griddle  requires  6  amp. ;  a  chafing 
dish  will  take  4  amp.  and  small  tea  kettles  require  from  4  amp.  to  / 
amp. ;  an  immersion  coil  for  cooking  food,  boiling  water  and  for 
heating  water  for  special  work  takes  from  4  amp.  to  8  amp,  accord- 
ing to  size ;  a  heating  pad  for  application  to  the  body  uses  only  .4 
amp.,  which  is  the  same  current  required  for  a  curling  iron.  Solder- 


112 


ELECTRICAL   CATECHISM. 


ing  irons  take  I  amp.  to  2  amp.,  according  to  size.  The  above  cur- 
rents are  taken  by  heating  devices  on  no-volt  circuits.  For  other 
voltage  the  resistances  are  proportioned  to  absorb  an  equivalent 
amount  of  energy.  (See  Nos.  213  and  218.) 

457.     How  much  does  it  cost  to  cook  by  electricity  f 

The  accompanying  table  gives  the  reported  cost  of  cooking  a 

luncheon  for  four  or  six  persons,  current  being  supplied  at  a  cost  of 

3.75  cents  per  kw-hour,  an  unusually  low  rate. 

COST  OF  COOKING  BY  ELECTRICITY. 


Amp. 
Current 

Minutes 

Cost 

Cooking  3  pounds  of  beef 

8.7 

2.6 

8.8 
8.8 
0.6 
3-i 
5-5 
5-5 
5- 

20 

154 
22 
20 
I 

14 
10 

5 
25 

1.186  cents. 

2.75 
1-33 

1.  21 
0.04 
0.29 

0-43 
0.19 

0.86 

Keeping  beef  warm            .. 

Parboiling  i  cauliflower  .  .        ... 

Baking  the  cauliflower  

Egg  sauce  for  cauliflower  

Keeping  above  warm  

Broiling  4  cutlets           

Heated  pan  too  soon  (wasted)..  .  . 
Frying  potatoes  in  butter  

Total  cost  of  current  for  cook- 
ing lunch  eon  for  four  



.... 

8.  29   cents. 

An  excellent  luncheon  for  six,  including  a  steak,  fish  with  tomato 
sauce,  potatoes  and  rice  a  la  conde  was  cooked  at  a  cost  for  current 
of  only  6.91  cents.  To  heat  water  for  washing  dishes  and  utensils 
the  cost  is  given  as  1.75  cents  for  2.5  gallons.  A  quart  of  coffee  can 
be  made  for  0.5  cent.  The  cost  to  a  family  of  four  for  three  ordinary 
meals  is  calculated  to  be  about  n  to  12.5  cents  per  day.  Electrical 
energy  generally  costs  more  than  3.75  cents,  and  the  above  costs 
would  vary  accordingly. 

458.     What  are  the  advantages  of  electric  heat  in  the  home? 

Electric  heat  is  best  adapted  to  lighting,  cooking,  ironing  and  mis- 
cellaneous uses  where  the  heat  can  be  localized.  It  avoids  the  vitia- 
tion of  air  and  the  danger  of  fire  incident  to  matches,  kerosene  and 
gas.  The  devices  are  easily  portable,  do  not  leak  or  smell,  are 
easily  and  instantly  turned  on  and  off.  Electric  heat  is  practicable  for 
warming  air  in  small  apartments.  Electric  fans  are  helpful  auxili- 
aries to  warm-air  furnaces  by  exerting  pressure  in  the  intake  or 
suction  at  inefficient  outlets ;  they  improve  circulation  about  radi- 
ators. They  increase  comfort  and  efficiency  by  circulating  air  in 
hot  weather  and  are  invaluable  in  hospitals. 


HEATING    EFFECTS. 


113 


FIG.  458.— ELECTRIC  LAUNDRY. 

459.  How  may  one  calculate  the  watts  necessary  to  raise  water 
a  certain  number  of  degrees  in  temperature? 

A  horse-power  is  550  ft.-lbs.  per  second,  and  also  746  joules  per 
second,  or  746  watts ;  therefore,  a  watt  is  .737,  and  a  kilowatt  737, 
ft.-lbs.  per  second.  A  heat  unit  is  equivalent  to  778  ft.-lbs. ;  there- 
fore, a  kilowatt  will  raise  a  pound  of  water  .94  deg.  F.  in  one  second, 
or  56.4  degs.  in  one  minute  ;  or  it  will  raise  18  Ibs.  of  water  from  the 
freezing  point  to  the  boiling  point  in  one  hour. 

460.  How  can  one  make  a  small  electric  heater  for  heating  coffee? 

Coil  iron,  steel  or  German  silver  wire  on  an  asbestos  pad,  insulat- 
ing the  convolutions  with  asbestos  thread.  Iron  and  steel  wire  have 
about  six  times  the  resistance  of  copper  wire,  and  German  silver 
wire  about  twelve  times.  The  lengths  per  ohm  of  copper  wire  at 
170  degs.  F.  are  80,  63,  50  and  40  ft.,  respectively,  for  Nos.  20,  21,  22 
and  23  wire.  From  the  above  data  the  length  of  wire  for  any  given 
current  can  be  found.  As  an  example,  for  No.  22  iron  wire  and  5 
amp.  on  a  no- volt  circuit,  about  220  ft.  of  wire  will  be  needed. 

461.  What  length  of  No.  22  German  silver  wire,  carrying  about 
5  amp.,  will  be  required  to  heat  an  electric  soldering  iron  on  a  500- 
volt  circuit?    Would  it  be   better  to  connect   two   or  three  irons 
in  series? 

The  amount  of  current  necessarily  depends  upon  the  size  of  the 
iron.  100  to  200  watts  will  heat  an  iron  weighing  three-fourths  of 
a  pound.  To  use  No.  22  German  silver  wire  and  a  voltage  of  500, 


114  ELECTRICAL   CATECHISM. 

one  will  need  about  1250  ohms,  which  will  be  in  the  neighborhood  of 
3000  ft.,  according  to  the  specific  resistance  of  the  wire  used.  No.  22 
wire  would  be  too  large  for  such  a  voltage,  as  the  coil  would  be  bulky 


FIG.  461.— ELECTRIC  SOLDERING  BOLT. 

and  weigh  considerably  over  4  Ibs.  One  thousand  feet  of  No.  27 
would  give  the  same  heat  and  weigh  much  less.  It  is  not  conven- 
ient to  connect  the  irons  in  series,  as  all  of  them  would  be  heated, 
when,  perhaps,  but  one  was  to  be  used. 

462.  Will  the  increase  of  temperature  of  a  generator  be  the  same 
whatever  the  normal  temperature  is  in  the  room,  i.  e.,  will  a  machine 
with  a  given  load  rise  72  deg.  F.  above  the  room  when  the  room 
is  at  60  deg.  F.  and  when  it  is  at  107  deg.  F.,  respectively? 

A  machine  with  a  given  load  will  rise  to  practically  the  same  num- 
ber of  degrees  above  the  atmospheric  temperature,  no  matter  what 
the  latter  is.  If  a  machine  loaded  in  a  room  at  60  deg.  F.  rises  to 
132  deg.  F.,  it  will,  with  the  same  load,  in  a  room  at  107  deg.  F., 
rise  to  about  180  deg.  F.  At  the  higher  initial  temperatures  the 
temperature  rise  will  be  a  little  greater  owing  to  the  fact  that  a  given 
number  of  cubic  feet  of  air  blown  through  the  machine  will  not  rep- 
resent quite  so  many  pounds  of  air,  since,  at  higher  temperatures,  it  is 
expanded  to  a  greater  volume. 

463.  What  is  the  formula  for  determining  the  current  that  will 
melt  copper  wire? 

Hospitalier  gives  the  formula  C  —  80  df  ,  where  the  diameter,  d, 
is  expressed  in  millimeters.  This  reduces  to  C  =  .325  df  when  the 
diameter  is  expressed  in  mils  or  thousandths  of  an  inch.  This  for- 
mula gives  for  the  melting  currents  of  Nos.  16,  22,  24  and  30  cop- 
per wire,  333,  117,  41,  29  and  12  amp.,  respectively.  This  formula 
can  not  be  relied  upon  for  fuses,  as  it  does  not  take  into  consideration 
the  cooling  effect  of  the  terminals.  For  copper  fuses  the  melting 
point  should  be  determined  for  the  style  of  block  used,  and  for,  say, 
one  or  two  sizes  of  wire,  combinations  of  which  can  then  be  used  for 
larger  currents.  An  approximate  rule  is  that  the  fusing  current  is 
about  fifteen  times  the  safe  current  allowed  by  the  insurance  rules. 

464.  How  can  electricity  be  used  to  thaw  frozen  water  pipes ? 
The  best  source  of  current  is  an  alternating  system,  especially  as 

the  temporary  wiring  for  this  purpose  can  be  put  in  the  primary  cir- 


HEATING  EFFECTS. 


115 


cuits,  the  transformers  being  located  close  to  the  frozen  pipes,  and 
comparatively  small  wires  can  therefore  be  used.  A  secondary  pres- 
sure of  50  volts  will  be  sufficient  for  all  ordinary  cases.  The  gen- 
eral arrangement  of  the  circuits  is  best  shown  in  the  accompanying 
drawing,  representing  a  frozen  service  from  a  street  main.  One  lead 
from  the  secondary  of  the  transformer  is  connected  to  the  piping  in- 
side the  dwelling  house  or  other  structure  to  which  the  service  leads, 
and  the  other  terminal  of  the  transformer  secondary  is  connected 


FIG.  464.— THAWING  FROZEN  PIPE. 

through  a  water  rheostat  with  the  street  main,  either  via  a  con- 
venient hydrant  or  a  service  branch  to  neighboring  premises  or  any 
other  point  of  access.  Care  should  be  taken  that  the  water  piping 
within  the  house  affected  is  not  crossed  with  other  pipes,  such  as  gas 
pipes,  etc.,  which  might  give  another  outlet  to  the  street  mains.  If 
the  piping  of  the  premises  is  elaborate  and  heavily  grounded,  it  would 
generally  prove  best  to  cut  it  off  from  the  service.  The  following 
reports  give  some  idea  of  the  amount  of  current  necessary  for  differ- 
ent cases  :  A  house  connection  of  80  ft.  of  i-in.  pipe  was  thawed  .with 
1 80  amp.  applied  for  fourteen  minutes.  Current  was  taken  from  a 
loo-kw  220- volt  direct-current  generator,  and  controlled  by  a  water 
rheostat  made  of  an  ordinary  barrel  filled  with  salt  water,  a  coil  of 
bare  copper  wire  in  its  bottom  forming  one  terminal,  and  a  bundle 
of  seven  common  arc  lamp  carbons  hung  from  a  piece  of  line  wire 
forming  the  upper  adjustable  terminal.  A  circuit  containing i  loft,  of 
I -in.  lead  pipe  and  45  ft.  of  6-in.  iron  pipe  was  thawed  by  155  amp. 
at  25  volts  in  seven  minutes.  Alternating  current  was  used  in  this 
case  and  controlled  by  a  reactive  coil  in  the  primary  circuit.  In  an- 
other case  a  f-in.  lead  pipe  required  190  amp.,  the  voltage  necessary 
to  force  this  through  85  ft.  of  the  lead  pipe,  and  22  ft.  of  6-in.  iron 
pipe  being  30  volts.  A  6-in.  main  320  ft.  long  required  350  amp. 
at  100  volts.  A  4-in.  cast-iron  main  was  thawed  by  a  current  of  160 
amp.  at  9  volts,  maintained  for  five  hours  and  forty  minutes. 


116 


ELECTRICAL  CATECHISM. 


465.  How  do  dentists  use  electric  heat? 

Dentists  use  heating  effects  of  current  for  ordinary  lighting  (see 
Nos.  467  to  481)  and  also  for  special  mouth  lamps  for  diagnosing 
dead  teeth  and  approximal  cavities  and  for  finding  orifices  of  canals. 
They  use  cauteries  for  cutting  off  overhanging  gum  tissue  without 
hemorrhage,  for  arresting  hemorrhage  after  extraction  and  to  stimu- 
late healing  of  old  sockets.  They  use  heated  tool-points  for  manipu- 
lating gutta-percha  and  wax,  for  drying  root  canals  and  for  assisting 
pyrozone  bleaching.  The  air  syringe  with  electrical  heater  is  used 
for  desiccating  purposes,  for  destroying  septic  matter  in  canals  and 
dentine  of  pulpless  teeth  and  for  obtunding  sensitive  dentine.  The 
electric  annealer  is  used  for  heating  and  annealing  gold,  and  some- 
times for  softening  gutta-percha.  The  electric  oven  is  without  rival 
for  baking  porcelain  work.  Electric  water  heaters  and  sterilizers 
have  great  hygienic  value.  Fuses  should  always  be  used  for  pro- 
tecting apparatus. 

466.  Hoiv  do  surgeons  use  electric  heat? 

They  use  it  principally  for  cautery  purposes.  Platinum  wires  are 
heated  by  electric  current,  and  applied  to  parts  to  be  burned  off  or 


FIG.  466A.-CAUTERY  ELECTRODES. 

to  be  seared.  By  heating  the  cautery  wire  white  hot  and  pressing  it 
against  fleshy  tissue  it  acts  like  a  knife  and  makes  a  smooth  cut.  By 
applying  a  lower  temperature  the  wire  will  sear  the  parts  and  form 
a  sort  of  scab  for  stopping  hemorrhages,  etc.  Cauteries  are  made 
in  various  shapes,  some  being  arranged  like  a  snare  to  be  placed 


HEATING    EFFECTS. 


11? 


around  some  undesirable  growth,  which  may  then  be  cut  off  by 
tightening  the  loop  while  the  wire  is  incandescent.     Electric  cautery 


FIG.  466B.— ELECTRIC  CAUTERY  SNARE. 

is  applicable  in  many  places  where  the  knife  would  be  used  with 
great  difficulty,  and  the  wounds  heal  rapidly  with  less  danger  from 
after  complications.  Cauteries  require  from  2  amp.  to  30  amp. 

467.     How  do  physicians  use  electric  heat? 

They  use  it  to  some  extent  for  heating  poultices  and  pads  to  be 


FIG.  467A.-ENDOSCOPIC  LAMP. 


FIG.  467B.-ELECTRIC  POULTICE. 


applied  to  various  parts  of  the  body.  They  also  use  it  for  small  in- 
candescent lamps,  which  may  be  introduced  into  or  near  various 
parts  of  the  body  for  illuminating  purposes. 

468.  How  is  the  heating  effect  of  the  current  used  for  lighting? 
Current  heats  to  high  temperatures  a  high  resistance  conductor,  as 

in  the  incandescent  lamp ;  or  it  vaporizes  the  ends  of  solid  conductors, 
the  light  coming  from  their  ends  as  in  the  ordinary  arc  lamp,  or  from 
the  vapor  as  in  the  flaming  arc ;  or  vaporizes  and  incandesces  mercury. 

469.  What  are  the  parts  of  an  incandescent  lamp? 
Excepting  the  Nernst  lamp,  the  light  comes  from  a  filament  of 

carbon,  platinum,  tantalum  or  tungsten  in  a  vacuum  within  a  glass 
globe;  current  enters  and  leaves  through  copper  and  platinum  con- 
ductors. The  glower  of  the  Nernst  lamp  consists  of  earthy  material 
similar  to  that  of  a  Welsbach  mantle ;  it  must  be  heated,  usually  by 


118 


ELECTRICAL  CATECHISM. 


an  automatic  electric  heater,  before   it  becomes  a  conductor;   no 
vacuum  is  used. 


FIG.    469 A.— CARBON,    TANTALUM    AND    TUNGSTEN    INCANDESCENT    LAMPS. 

470.     Hoitf  are  incandescent  lamps  usually  operated? 
They  are  generally  connected  in  multiple  between  wires  having  a 
nearly  constant  difference  of  potential.     For  special  purposes,  such 


HOLDING  SCREW 

ALUMINUM  PLUG 

I ARMATURE  SUPPORT 

L.POST 

, BALLAST 

CUT  OUT  COIL 

ARMATURE 
~-T-^:8ILVER  CONTACT  STOP 

• HOUSING 

CONTACT  SLEEVE  PORCELAIN 

GLOBE  HOLDING  SCREW 

' HOLDER  PORCELAIN  )[j 

HEATER  PORCELAINf  C 

HEATER  TUBEfJ 

1 GLOWER  '  3 


QLOBE 


FIG.    469B.— NERNST    LAMP,    PARTS    AND    CIRCUITS. 

as  lighting  electric  street  cars  or  for  lighting  streets,  a  number  of 
lamps  are  connected  in  series  across  lines  having  500  to  1000  volts, 
or  even  higher.  (See  Figs.  336  and  337.) 


SEATING  EFFECTS.  119 

471.  What  are  the  common  voltages  at  which  incandescent  lamps 
are  operated? 

It  was  formerly  common  to  use  from  50  volts  to  55  volts  with  lamps 
lighted  by  alternating  currents,  but  it  is  now  more  common  to  use 
from  100  volts  to  120  volts,  or  from  220  volts  to  250  volts.  Lamps 
on  electrically  lighted  railway  cars  take  30,  80,  96  or  1 10  volts.  Lamps 


L_j)  Dynamo 


Lamps 


FIG.  470.— INCANDESCENT  LIGHT  CIRCUIT. 

to  be  operated  by  current  from  batteries  are  made  for  3  volts  and 
upward.  It  is  customary  to  have  all  the  lamps  on  an  installation  for 
the  same  voltage,  and  new  lamps  should  be  ordered  for  the  voltage 
regularly  maintained.  If  the  lamps  are  of  too  high  voltage,  they 
will  not  light  up  with  sufficient  brilliancy,  the  filaments  being  only 
dull  red  or  yellow.  If  the  lamps  are  of  too  low  voltage,  they  will 
take  too  much  current  and  will  light  up  brilliantly,  but  will  quickly 
burn  out.  As  the  lamps  get  old,  they  get  dim,  and  sometimes  it  is 
practicable  to  move  the  old  and  dim  lamps  to  positions  nearer  the 
dynamo,  where  the  voltage  is  somewhat  higher. 

472.  What  is  the  relation  between  voltage  and  candle-power? 

The  candle-power  of  the  light  increases  much  faster  than  the  volt- 
age. For  example,  a  carbon  lamp  intended  to  give  16  cp  at  105 
volts  will  give  12.5  at  103  volts  or  19  at  107  volts. 

473.  What  is  meant  by  the  efficiency  of  an  incandescent  lamp? 

The  efficiency  of  an  incandescent  lamp  usually  refers  to  the  num- 
ber of  watts  absorbed  per  candle-power.  For  example,  a  4-watt 
i6-cp  lamp  would  require  four  times  sixteen,  or  64  watts.  The 
practice  in  America  is  to  use  carbon  filament  lamps  taking  3.5  or  3.1 
watts  per  candle,  "  metallized  carbon  "  lamps  of  2.5  watts,  tantalum 
lamps  of  2  watts  and  tungsten  lamps  of  1.25  watts  per  candle-power. 
Low  voltage  lamps  for  batteries  take  less  energy  per  candle-power. 

474.  What  determines  the  best  efficiency  to  use? 

The  cost  of  power  and  lamps,  and  the  closeness  of  regulation  of 


120 


ELECTRICAL  CATECHISM. 


the  voltage  are  the  principal  elements.  The  more  costly  the  power 
and  the  cheaper  the  lamps,  the  higher  should  be  the  efficiency  of  the 
lamps.  The  life  of  the  lamps  is  shorter  when  operated  at  high 
efficiency,  and  the  life  of  the  lamps  is  rapidly  shortened,  if  the  volt- 
age rises  above  the  normal,  so  that  the  voltage  should  be  regulated 
so  as  to  be  as  uniform  as  possible.  In  plants  where  the  power  is 
unsteady,  or  where  there  is  much  variation  in  voltage  on  account  of 
line  losses,  it  is  not  economical  to  use  high  efficiency  lamps.  On  110- 
volt  circuits  3.i-watt  lamps  are  suitable  only  where  the  voltage  never 
varies  more  than  2  volts ;  3.5-watt  lamps  are  suitable  for  ordinarily 
well  regulated  plants,  while  4-watt  lamps  are  usually  best  where 
regulation  is  poor.  (See  also  No.  473.) 

475.     To  what  extent  does  the  voltage  affect  the  life  of  an  in- 
candescent lamp? 

This  is  shown  in  the  following  table : 


|     Per  Cent  of 
(Normal  Voltage 

Life  Factor 

Per  Cent  of 

Normal  Volt'g 

Life  Factor 

Per  Cent  of 
Normal  Volt'g 

Life  Factor 

| 
IOO 

I.OOO 

103 

.562 

10=; 

•374 

101 

.818 

104 

•452 

106 

.310 

102 

.681 

From  this  it  is  seen  that  the  life  of  the  lamp  is  halved  by  an  in- 
crease of  3  per  cent,  and  is  reduced  two-thirds  by  an  increase  of  6 
per  cent  in  the  voltage. 

476.  How  are  the  candle-poiver  and  efficiency  of  the  lamp  af- 
fected by  variations  of  voltage ? 

This  is  shown  in  the  following  table  published  by  the  General 
Electric  Company: 


Per  Cent  of 

Per  Cent  of 

Efficiency  in 

Per  Cent  of 

Per  Cent  of 

Efficiency  in 

A  ormal 
Voltage 

Normal 
Can  die-  Power 

Watt*  per    - 
Candle 

Normal 
Voltage 

Normal 
Candle-Power 

Watts  per 
Candle 

90 

53 

4.68 

99 

94-5 

3-22 

91 

57 

4.46 

IOO 

IOO 

3-1 

92 

61 

4.26 

101 

106 

2-99 

93 

65 

4.1 

102 

112 

2.9 

94 

69.5 

392 

103 

118 

2.8 

95 

74 

3.76 

104 

124.5 

2.7 

96 

79 

3-6 

105 

131-5 

2.62 

97 

84 

3-45 

1  06 

138.5 

2.54 

98 

89 

3-34 

HEATING   EFFECTS. 


121 


4.7 

4.5 
4.3 
4.1 
S  3.9 
<§  3.7 
I  3.5 

3.1 
2.9 
2.7 

2.5 
1( 

/ 

/ 

S 

/ 

N 

SQf( 

J 

$/ 

^* 

<*/ 

pX 

\ 

X 

2 

$ 

^ 

^ 

X 

s 

r4 

X 

^ 

^x 

s 

^ 

x 

^x 

^ 

*^s 

^ 

x^ 

^ 

**++ 

^^ 

J6    105    104    103    102    101    100     99      98      97     96      95     94      93      92      91      9( 

„  Per  cent  of  Normal  Voltage 

FIG.  476.— VARIATION  OF  CANDLE  POWER  AND  EFFICIENCY 
WITH  VOLTAGE. 

A  lamp  giving  16  cp  with  3.1  watts  per  candle  at  105  volts  will 
give  89  per  cent  of  the  light,  or  14.5  candles,  at  98  per  cent  of  normal 
voltage,  or  103  volts,  with  3.34  watts  per  candle. 

477.  What  is  meant  by  the  smashing  point  of  an  incandescent 
lamp  ? 

As  a  lamp  gets  old  its  candle-power  and  efficiency  drop  off,  so  that 
after  a  certain  point  it  becomes  cheaper  to  throw  the  lamp  away,  or 
to  smash  it  and  get  a  new  lamp,  than  it  is  to  continue  burning  the 
old  one.  The  smashing  point  is  generally  reached  when  the  candle- 
power  of  the  lamp  has  fallen  to  80  per  cent  of  its  initial  value. 

478.  How  does  the  candle-power  of  an  incandescent  lamp  vary 
with  age? 

The  candle-power  generally  increases  for  a  time,  and  then  begins 
to  fall,  reaching  the  initial  value  at  about  100  hours.  The  rate  at 
which  the  candle-power  falls  varies  with  lamps  of  different  makers, 
and  also  with  different  lamps  from  the  same  factory.  As  the 
efficiency  rises  the  candle-power  falls  off  more  rapidly.  The  accom- 
panying diagram  shows  results  of  careful  experiments  with  an  excel- 
lent lamp,  curve  I  showing  the  percentage  of  original  candle-power 
after  burning  various  lengths  of  time  when  started  at  an  efficiency  of 
4  watts  per  candle,  and  kept  constantly  at  the  same  voltage ;  curve 
2  shows  a  similar  lamp  started  at  an  efficiency  of  3.5  watts  per 


ELECTRICAL  CATECHISM. 


candle ;   curves  3  and  4  show  similar  lamps  started  at  3  watts  and 
2.5  watts  per  candle. 


200  3OO  *OO 


BOO  60O 


HOURS 

FIG.  478.-LIFE  AND  EFFICIENCY  OF  INCANDESCENT  LAMPS. 

479.     What  is  meant  by  a  target  or  shotgun  diagram? 

A  target  diagram  shows  the  watts  absorbed  and  the  candle-power 
of  a  number  of  lamps  which  are  tested  for  uniformity.  Such  a  test 
should  be  made  for  each  lot  of  new  lamps  bought.  The  lamps  are 
each  placed  on  a  photometer  and  subjected  to  the  voltage  marked  on 
the  lamp  base,  and  the  watts  and  candle-power  measured  at  the  same 
time.  The  readings  for  each  lamp  are  recorded  and  also  plotted  on  a 


4SI4BI4!? 


FIG.  479.— TARGET  DIAGRAM. 

sheet.     The  record  of  a  test  of  a  poorly  sorted  lot  of  lamps  is  shown 
in  the  accompanying  target  diagram. 

480.     What  is  a  photometer? 

The  photometer  is  an  apparatus  for  measuring  light.  The  lamp 
to  be  tested  is  placed  at  one  end  of  a  bar,  a  standard  lamp  of  known 
candle-power  is  placed  at  the  other  end,  and  each  is  subjected  to  the 
voltage  specified.  A  movable  screen  or  "sight  box"  is  then  moved 
along  the  bar  between  the  two  lights  until  its  two  sides  are  equally 


HEATING    EFFECTS.  123 

lighted  by  the  two  lamps.  When  a  balance  is  found,  the  strengths 
of  the  two  lights  are  directly  as  the  square  of  the  distances  from 
each  light  to  the  sight  box  screen.  At  the  same  time  the  voits  and 


FIG.  480.— PHOTOMETER. 

amperes  are  measured  for  the  lamp  under  test,  and  the  watts  are 
calculated  by  multiplying  volts  by  amperes.  Instead  of  an  incan- 
descent lamp,  an  oil  or  gas  lamp  is  sometimes  used  as  a  working 
standard,  as  shown  in  the  figure. 

481.  What  is  the  temperature  of  the  filament  in  an  incandescent 
lamp? 

A  carbon  filament  runs  at  1700  to  2100  degrees  centigrade.  If  the 
voltage  is  too  high  the  lamp  takes  too  much  current  and  the  tem- 
perature of  the  filament  becomes  so  high  that  it  becomes  soft  and 
droops  until  it  may  touch  the  glass  bulb  which  then  cracks  and  allows 
air  to  enter  and  burn  the  filament.  An  abnormally  high  temperature 
also  causes  disintegration  of  the  filament  and  causes  its  candle-power 
to  drop  off  rapidly.  Tantalum  and  tungsten  filaments  run  hotter. 

482.  How  many  incandescent  lamps  are  in  use? 

Probably  between  one  and  two  hundred  million.  The  factories 
of  one  company  are  stated  to  have  a  producing  capacity  of  60,000,- 
ooo  lamps  annually.  Within  the  first  two  years  of  its  introduction, 
something  like  1,200,000  tantalum  lamps  were  sold  in  America. 

483.  What  is  the  electric  arc? 

The  electric  arc  is  a  phenomenon  discovered  by  Davy  about  1800. 
He  sent  a  strong  current  through  two  carbon  pencils  and  then  sep- 
arated them.  The  current  continued  to  pass,  raising  the  tempera- 
ture of  the  ends  of  the  carbons  to  a  high  degree,  while  a  vapor  of 
carbon  passed  between  the  carbon  pencils.  By  reason  of  the  rising 
currents  of  air,  the  stream  of  incandescent  vapor  between  the  pencils 
assumed  an  arched  form,  and  Davy  named  it  the  electric  arc  or  arch. 
Later  experiments  show  that  the  end  of  the  positive  carbon  is  raised 
to  the  temperature  of  volatilization,  that  of  the  arc  itself  being  higher 
and  that  of  the  negative  carbon  being  lower.  (See  No.  494.) 


124: 


ELECTRICAL  CATECHISM. 


484.     What  is  the  source  of  light  from  an  arc? 

Light  comes  from  the  ends  of  the  electrodes  or  pencils,  the  prin- 
cipal source  of  light  from  the  open  carbon  arc  lamps ;  it  comes 
from  the  arc  stream  as  an  incandescent  gas,  the  principal  source  of 
light  from  the  flaming  arcs ;  it  comes  from  the  incandescence  of 
solid  particles  entangled  in  the  outer  envelop  of  the  gas  stream ;  these 
particles  may  give  selective  radiation,  that  is,  more  than  the  usual 
proportion  of  the  energy  radiated  by  them  may  be  of  luminous  wave- 
length. 


FIG.  489.— AN   OPEN   ARC  LAMP. 

485.     What  currents  and  voltages  are  used  by  arc  lamps? 

Lamps  for  street  lighting  are  generally  operated  on  series  circuits 
supplied  with  constant  current.  The  early  "  low  tension  "  lamps 
took  20  amperes  at  25  to  40  volts ;  the  "  high  tension  "  lamps  with  open 
arcs  took  45  to  50  volts  on  9.6  to  10  amperes  for  "  full  "  or  "  2000 
nominal  candle  "  lights,  and  6  or  6.8  amperes  for  "  half  "  or  "  1200 
candle  "  lights.  The  enclosed  arcs  take  75  to  80  volts  on  5  to  6.6 
amperes  of  direct  current  or  6.6  to  7.5  amperes  of  alternating  cur- 
rent. For  interior  illumination  it  is  customary  to  use  enclosed  arcs 
adjusted  for  about  80  volts  at  the  arc  and  taking  2.5  to  5  amperes  of 
direct  current  or  4  to  7.5  amperes  of  alternating  current;  the  en- 
closed lamps  are  used  singly  on  no-  to  I25~volt  circuits,  two  in 
series  on  220-  to  25O-volt  circuits  and  five  in  series  on  5oo-volt  cir- 
cuits, part  of  the  line  voltage  being  taken  up  by  steadying  resistances 
or  alternating  current  choke  coils.  Flaming  carbon  arc  lamps  use  6 
to  12  amperes  with  40  to  45  volts  at  the  arc  with  direct  current  or 


HEATING  EFFECTS. 


125 


38  to  40  with  alternating  current.  Metallic  flame  or  "  magnetite  " 
lamps  take  4  amperes  direct  current  at  75  to  80  volts  at  the  arc. 
Focusing  lamps,  used  for  lanterns,  spot  lights  and  searchlights, 
take  from  10  to  150  amperes. 

486.     What  is  the  candle-power  of  an  arc  light? 

An  open  carbon  arc  lamp  with  45  volts  at  10  amperes  has  1200  to 
2000  cp  in  a  direction  about  40  degrees  below  horizontal,  300  to  400 
cp  horizontally  and  an  average  of  about  600  cp  over  the  lower  hemi- 
sphere. The  6.8  ampere  open  arc  gives  about  60  per  cent  of  the 
above  values.  An  enclosed  arc  lamp  taking  about  5  amperes  direct 
current  from  a  i  lo-volt  circuit  gives  about  320  cp  30  to  40  degrees 


FIG.    490.— AN    ENCLOSED   ARC   LAMP. 

below  the  horizontal,  about  230  horizontally  and  an  average  of  about 
240  over  the  lower  hemisphere.  Alternating  current  arcs  give  about 
25  per  cent  less  light  than  direct-current  arcs  taking  equal  energy. 
A  flaming  arc  taking  10  amperes  with  46  volts  at  the  arc  gives  about 
1500  horizontal  cp  and  an  average  of  about  3000  over  the  lower 
hemisphere.  A  magnetite. arc  taking  4  amperes  and  80  volts  gives 
about  550  horizontal  cp  and  an  average  of  about  370  over  the  lower 
hemisphere.  A  direct-current  mercury-vapor  tube  taking  385  watts 
gives  a  maximum  of  about  730  cp  directly  underneath  and  an  aver- 
age of  400  to  750  over  the  lower  hemisphere. 

487.     Hozv  long  do  the  electrodes  last  in  arc  lamps? 

In  a  9.6  ampere  direct-current  open  arc,  a  positive  carbon   12 


126 


ELECTRICAL  CATECHISM. 


inches  long  and  0.5  inch  in'  diameter  lasts  from  7  to  12  hours,  a  nega- 
tive of  equal  size  lasting  from  one  to  three  times  as  long;  for  all- 
night  operation  it  is  necessary  to  have  a  double  lamp.  The  carbons 
in  enclosed  lamps  last  from  80  to  200  hours.  Flaming  arcs  need  re- 
trimming  or  re-carboning  after  10  to  20  hours.  The  metallic  flam- 
ing or  magnetite  lamps  require  trimming  about  every  150  hours. 

488.     How  are  the  arc  electrodes  kept  the  right  distance  apart? 

Each  lamp  has  a  mechanism,  usually  driven  by  gravity  and  con- 
trolled by  electromagnets.  A  coil  in  series  with  the  arc  and  carry- 
ing the  whole  current  is  generally  used  to  move  the  electrodes  apart, 
and  a  coil  of  comparatively  high  resistance  shunted  around  the  arc 
commonly  feeds  the  electrodes  together.  When  a  single  lamp  is 
operated  across  a  constant  potential  circuit,  only  one  coil  is  necessary, 
a  series  coil  being  used  in  lamps  arranged  so  that  the  electrodes  are 


+  Lead 


Seal  off 


Condensing  Chamber 


vlrott  Electrode 
Platinum  Wire 
Porcelain  Lamp  Tip 


Vacuum  Tube. 


Mercury  Electrode 

Platinum  Wire 

Porcelain  Lamp  Tip 


FIG.    493.— MERCURY  VAPOR  LAMP. 

in  contact  when  no  current  is  passing,  a  shunt  coil  being  used  where 
the  electrodes  are  apart  when  no  current  passes.  For  lamps  on 
series  circuits,  a  cut-out  coil  is  necessary  to  prevent  excessive  voltage 
at  one  lamp  or  to  prevent  opening  the  entire  circuit  in  case  a  lamp 
fails  to  feed;  for  constant  current  circuits,  the  cut-out  coil  short- 
circuits  the  lamp;  when  several  lamps  are  operated  in  series  across 
constant  potential  mains,  the  lamp  cut-out  substitutes  an  equivalent 
dead  resistance  when  the  lamp  fails  to  feed,  or  else  the  entire 
"  string  "  of  lights  is  extinguished.  When  the  exact  position  of  the 
arc  is  unimportant,  only  one  electrode  is  moved,  this  being  the  posi- 
tive except  in  case  of  magnetite  or  "  metallic  flame  "  lamps.  The 
general  basis  of  arc  lamp  regulation  is  that  with  a  given  current, 
the  effective  resistance  of  the  arc  and  the  difference  of  potential  be- 
tween its  terminals  vary  with  the  length  of  the  arc;  consequently, 
the  longer  the  arc  the  more  current  will  pass  through  the  shunt  coil 


HEATING  EFFECTS.  127 

and  the  stronger  it  will  pull  its  armature  and  the  attached  mechanism. 
(See  Nos.  726  to  736.) 

489.  What  is  an  open  arc? 

In  the  open  arcs,  the  electrodes  are  of  nearly  pure  carbon,  a  glass 
globe  surrounding  the  arc  to  protect  it  from  wind.  The  positive 
electrode  is  above,  the  end  assuming  a  cup  shape  called  the  crater  and 
being  the  principal  source  of  light.  The  arc  stream  gives  little 
light,  simply  adding  a  violet  tinge  when  long. 

490.  What  is  an  enclosed  arc? 

With  the  enclosed  arc  lamps  a  small  glass  globe  surrounds  the  arc 
and  fits  the  carbon  pencils  so  closely  that  the  air  inside  the  globe  can 
change  only  slowly.  This  reduces  the  rate  of  combustion  of  the  car- 
bons, so  that  they  last  longer.  This  not  only  reduces  the  frequency 
of  retrimming  (recarboning)  the  lamp,  but  also  reduces  the  de- 
mands upon  the  feeding  mechanism  of  the  lamp.  The  arc  is  longer, 
the  crater  less  distinct  and  the  arc  less  efficient.  The  light  is  better 
distributed,  being  stronger  horizontally  and  without  intense  con- 
trasts below.  The  longer  life  of  the  carbons,  even  though  more 
expensive  in  first  cost,  saves  the  operating  company  from  $7  to  $10 
annually.  (See  also  Nos.  485  to  487.) 

491.  What  is  a  flaming  arc? 

The  flaming  arc  lamp  uses  electrodes  consisting  of  a  carbon  shell 
with  a  core  of  powdered  carbon,  mineral  salts  and  a  suitable  binder ; 
the  minerals  make  the  arc  highly  luminous,  so  that  the  total  amount 
of  light  is  about  treble  that  from  ordinary  carbons  using  equal 
energy.  On  account  of  the  fumes  and  residue  from  the  mineralized 
carbons,  the  arc  must  be  well  ventilated,  and  is  best  suited  for  ex- 
terior lighting.  The  electrodes  are  generally  inclined,  making  a 
narrow  "  V  "  and  throwing  most  of  the  light  downward,  the  arc 
being  deflected  by  an  electromagnet.  (See  Nos.  485  to  487  and  502.) 

492.  What  is  a  magnetite  or  metallic  flame  lamp? 

In  these  lamps,  the  negative  electrode  furnishes  the  material  for 
the  arc  vapor,  and  consists  of  a  thin  steel  tube  packed  with  oxides 
of  metals  such  as  iron  (magnetite),  titanium  and  chromium.  In 
vaporizing,  these  add  great  luminosity  to  the  arc;  they  leave  a  con- 
siderable amount  of  fluffy  soot  which  is  carried  away  by  special 
ventilation.  One  electrode  is  directly  above  the  other,  one  company 
having  the  positive  above,  another  the  negative. 

493.  What  is  the  mercury  vapor  lamp? 

The  Cooper-Hewitt  lamp  consists  essentially  of  a  glass  tube  hav- 


128  ELECTRICAL   CATECHISM. 

ing  at  each  end  a  bulb  containing  mercury  and  a  platinum  leading- 
in  wire,  the  air  being  exhausted.  A  tube  about  4  feet  long  and  i 
inch  in  diameter  takes  about  3  amperes  direct  current  at  about  no 
volts,  giving  a  strong  greenish  light.  The  lamp  is  commonly  started 
by  tipping  the  tube  until  a  thin  stream  of  liquid  mercury  makes  con- 
nection between  the  two  electrodes ;  when  the  tube  is  tilted  back,  an 
arc  is  started  which  fills  the  tube  with  ionized  mercury  vapor  which 
forms  a  luminous  path  between  the  lower  cathode  and  the  upper 
anode  (see  Nos.  607  and  644).  Special  devices  are  made  for  mak- 
ing the  lamp  self-starting,  even  without  tipping.  They  are  some- 
times used  with  alternating  currents. 

494.  What  are  the  temperatures  in  the  arc? 

There  is  some  variation  in  the  results  of  different  investigations, 
but  the  temperature  of  the  crater  at  the  end  of  the  positive  carbon 
is  between  3500  deg.  and  3900  deg.  C.  The  temperature  of  the 
end  of  the  negative  carbon  is  about  2500  deg.,  while  the  temperature 
of  the  arc  itself  is  reported  as  3900  deg.  to  4800  deg.  The  arc  has 
about  the  highest  temperature  obtainable. 

495.  Is  any  use  other  than  lighting  made  of  the  high  temperature 
of  the  arc? 

Various  processes  for  welding  metals  and  for  the  reduction  of 
metals  or  other  metallurgical  work  have  been  developed  both  from 
the  heat  of  the  arc  and  also  from  the  heat  from  the  ordinary  opera- 
tion of  the  current.  These  are  sometimes  classified  as  incandescent 
and  arc  processes. 

496.  What  different  processes  are  there  for  electric  welding? 
There  are  three  classes  of  processes :  that  of  Thomson,  who  sends 

a  heavy  current  through  the  two  pieces  whose  ends  are  placed  in  close 
contact  and  are  pressed  together  when  the  current  has  heated  their 
ends  to  a  welding  temperature ;  a  second  class  is  that  developed  by 
Bernardos  and  others,  who  use  the  arc  to  heat  the  metals  to  the  de- 
sired temperature ;  a  third  method  is  developed  from  the  "water  pail 
forge"  of  Lagrange  and  Hoho. 

497.  Describe  the  Thomson  welding  process. 

The  Thomson  welding  process  consists  in  clamping  together  the 
pieces  to  be  welded,  and  then  sending  a  heavy  current  through  the 
clamp  and  the  joint.  The  resistance  at  the  joint  is  higher  than  else- 
where, and  the  great  heat  developed  there  (see  Nos.  371  and  415) 
quickly  raises  the  joint  to  a  welding  temperature.  The  parts  are 
then  forced  together  mechanically,  completing  the  weld.  Most  of 
the  heat  is  developed  exactly  where  required,  part  being  carried  away 


HEATING    EFFECTS. 


129 


by  the  clamps  and  some  heat  being  developed  in  other  parts  of  the 
circuit.  The  process  is  economical  and  easily  competes  with  fire 
welding.  By  adjusting  the  current  strength,  pieces  of  any  size  may 
be  welded  without  burning.  Some  welds  easily  made  electrically 
are  difficult  or  impracticable  by  other  processes.  Alternating  cur- 
rent is  best  adapted  for  welding,  since  large  currents  at  low  voltage 
are  easily  generated  close  to  the  work  by  the  use  of  transformers  (see 
Nos.  1404  and  1405).  When  direct  current  only  is  available,  a  con- 
verter (see  Nos.  498  and  1510  to  1512)  supplies  alternating  cur- 
rent. The  welding  of  copper  requires  about  60,000  amperes  per 
square  inch  of  metal.  An  interesting  process  is  the  welding  of  rails 
for  electric  roads. 

498.     How  are  rails  electrically  welded? 

One  car  carries  a  rotary  transformer  which  changes  a  current  of 
about  275  amp.  at  about  500  volts  from  the  trolley  line  into  an  alter- 
nating current  of  300  volts.  The  alternating  current  passes  to  the 
welding  transformer,  which  is  hung  on  a  crane  behind  the  car.  The 


FIG.  498.— ELECTRIC  KAIL   WELDING. 

rails  to  be  welded  are  butted  against  each  other  and  two  chucks  are 
welded  from  either  side  to  the  ends  of  the  two  rails.  A  hydraulic 
jack  forces  the  chucks  against  the  cleaned  ends  of  the  rails  and  so 
completes  the  weld  after  the  current  has  supplied  the  necessary  heat. 
About  four  welds  can  be  prepared  and  finished  in  an  hour. 

499.  Is  the  heating  effect  of  the  current  used  for  metal  working 
other  than  welding? 

Thomson  has  used  heavy  currents  for  heating  and  annealing  spots 
in  steel  armor  plates  where  it  is  desired  to  drill  holes  or  to  do  other 


130  ELECTRICAL   CATECHISM. 

work.  The  heating  effect  of  the  current,  either  passing  through  the 
material  itself  or  through  an  oven  or  furnace  (see  No.  503),  is 
used  for  tempering  springs,  annealing,  heating  for  forging,  etc. 

500.  Describe  the  water  pail  forge. 

Lagrange  and  Hoho  found  that  if  an  electric  circuit  was  closed  by 
dipping  the  negative  terminal  into  a  conducting  liquid,  the  cathode 
or  negative  terminal  became  very  hot  and  melted  if  the  voltage  was 
above  125.  This  has  been  developed  by  Burton  and  others  into  what 
is  known  as  "hydro-electric  heating."  The  best  solution  is  ten  parts 
of  carbonate  of  soda  and  one  part  of  borax  dissolved  in  water  until 
the  specific  gravity  is  1.15  at  70  deg.  F.  Metals  can  be  heated  for 
welding,  or  the  heat  may  be  graduated  for  such  purposes  as  heating 
soldering  irons. 

501.  How  is  the  arc  used  for  welding? 

The  arc  welding  system  is  suitable  for  surface  work  and  has  been 
applied  by  Bernardos  and  others  for  such  work  as  filling  blow-holes 
in  castings  and  for  welding  thin  sheets  of  metal.  The  figure  shows  an 
application  of  this  method  to  the  welding  of  a  flange  on  a  pipe.  It 
will  be  noticed  that  the  material  upon  which  the  work  is  being  done 


FIG.  501.— BERNARDOS'  ARC  WELDING  PROCESS. 

is  connected  to  the  positive  terminal  of  the  circuit,  while  the  carbon 
points,  which  are  held  in  the  workman's  hands,  are  connected  through 
the  resistances,  rr',  to  the  negative  side  of  the  circuit.  This  opera- 
tion is  said  to  work  with  great  rapidity  and  satisfaction,  but  a  prac- 
tical point  of  much  importance  in  connection  with  it  is  to  heat  the 
metal  to  be  operated  upon  to  a  dull  red  heat  in  an  ordinary  fire  be- 
fore exposing  it  to  the  arc  temperature.  The  process  is  used  con- 
siderably on  the  Continent  for  the  repairing  of  boilers,  cracks  and 
fissures  in  steel  plates  being  easily  mended  in  this  manner  and  filled 


HEATING  EFFECTS.  131 

up  with  melted  metal.  Zerner  and  others  have  modified  this  method 
by  blowing  an  arc  against  the  surfaces  to  be  heated,  so  that  the  ma- 
terials acted  upon  do  not  become  part  of  the  electrical  circuit.  This 
is  sometimes  called  an  electric  blowpipe. 

502.     Describe  the  electric  blowpipe. 

Zerner's  blowpipe  is  illustrated  in  the  figure.  An  arc  at  a  poten- 
tial difference  of  about  85  volts  and  with  a  current  of  20  amp.  is  es- 
tablished between  the  carbons  C  and  C '.  The  two  coils,  S,  are  solen- 
oids, producing  a  magnetic  field  in  a  direction  at  right  angles  to 
that  of  the  arc,  causing  the  latter  to  be  projected  to  one  side,  as  shown 


FIG.  502.— ZERNER  ELECTRIC  BLOWPIPE. 

at  A.  The  regulation  of  the  carbons  is  effected  by  the  screw  V,  as 
clearly  shown  in  the  figure.  This  simple  apparatus  is  used  a  great 
deal  in  England,  especially  in  the  manufacture  of  bicycles  for  braz- 
ing the  tubes  of  the  frame,  and  in  work  on  thick  plates  of  steel,  as, 
for  example,  boiler  iron.  The  great  advantages  possessed  by  this 
form  of  instrument  are  at  once  evident,  as  it  is  easily  handled,  is  light, 
and  gives  an  exceedingly  high  temperature  and  at  a  very  definite 
point.  Both  the  Bernardos  and  the  Zerner  methods  are  successfully 
used  for  cutting  up  metals,  such  as  the  removal  of  iron  beams  and 
large  riveted  tanks  from  the  interior  of  buildings. 

503.     What  is  an  eleciric  furnace? 

An  electric  furnace  is  an  arrangement  for  obtaining  a  high  tem- 
perature in  an  inclosed  space  by  means  of  the  heat  from  an  electric 
current.  Electric  ovens  are  made  for  comparatively  low  tempera- 
tures, like  electric  heaters  (see  No.  455)  having  a  cavity  inside  and 
being  lagged  to  hold  in  the  heat.  For  higher  temperatures,  fur- 
naces are  made  on  both  the  arc  and  the  incandescent  principles  (see 
Nos.  468,  481,  494).  In  the  incandescent  or  resistance  type  of  fur- 
nace, the  material  is  heated  by  current  passing  either  through  it  or 
through  a  conducting  core  or  tube.  In  the  induction  furnace,  the 
material  heated  constitutes  an  entire  circuit  by  itself,  being  the  sec- 
ondary of  a  transformer  (see  Nos.  1404,  1405)-  In  the  arc 


132 


ELECTRICAL  CATECHISM. 


of  furnace,  the  material  is  either  passed  directly  through  the  arc 
stream  or  it  is  acted  upon  by  heat  conducted  from  the  arc.  Both 
arc  and  resistance  effects  are  used  in  some  furnaces,  and  some  add 
chemical  action  of  the  current. 

504.  How  is  the  electric  furnace  used? 

The  high  degree  of  heat  obtained  is  used  in  effecting  chemical 
combinations  of  various  materials.  In  some  cases  the  elements  o* 
the  different  ingredients  simply  combine  to  form  new  combinations 
At  the  high  temperatures  attained  in  some  electric  furnaces,  the  in- 
gredients dissociate,  that  is,  separate  so  that  the  elements  are  free 
to  form  new  combinations.  In  such  cases  the  current  is  simply  the 
source  of  heat  and  does  not  directly  enter  into  the  reactions.  In 
other  cases  the  current  not  only  furnishes  a  high  temperature,  but 
also  takes  direct  part  in  the  reactions,  both  the  heating  and  the 
chemical  effects  of  the  current  being  used. 

505.  Give  examples  of  furnaces  in  zvhich  these  effects  are  used. 
The  simple  heating  effect  of  the  current  without  any  chemical 

action  is  used  in  furnaces  for  making  artificial  graphite  from  carbon 
and  artificial  corundum  ("alundum")  from  bauxite  (oxide  of 
aluminum),  etc.  The  heating  effect  as  a  means  of  inducing  chemi- 
cal action  is  used  in  furnaces  for  making  calcium  carbide,  carborun- 
dum (carbide  of  silicon),  carbon  bisulphide,  phosphorus,  arsenic, 
cyanides,  siloxicon,  ferro-alloys,  etc. ;  for  fixing  atmospheric  nitrogen, 
smelting  ores  and  refining  metals.  The  combined  heating  and 
chemical  effects  of  the  current  are  used  in  the  production  of  alumi- 
num, caustic  soda,  sodium,  etc. 


Name. 

Formula 

Discoverer 

Remarks. 

Aluminum  , 

AF4C3 
BaCn 

Moissan,   1894 
"         1894 

Water  evolves  methane 
*'          "      acetylene 

Calcium  .  .  

CaCa 

Wohler,    1862 

Cerium 

CeaC 

Moissan,   1896 

"         ethylene  and  methane 

V 

rt 
* 

fHucinum  
Lanthanum   

G14C8 
LaCa 
LioCo 

Lebeau,     1895 
Moissan,    1896 
"         1896 

"   -  methane 
acetylene,  ethylene  and  methane 
"          **      acetylene 

£ 
15  - 
I 

Manganese  

Potassium  
Sodium  ....     

Mn3C 

K2Ca 
Na,Co 

Troost  and 
Hautefeiulle 
Davy,        1808 
Berthelot,  1866 

methane  and  hydtogen 
acetylene 

8 

Strontium  ...... 
Thorium....  .... 

SrCa' 
ThCa 

Moissan,    1894 
(  Moissan,   jg96 

acetylene,  hydrogen  and  hydro- 

£ 

UroCa 

j  Etard, 
Moissan    1896 

carbons 
Water  evolves  acetvlene,  methane,  hydrogen 

Ytrium 

YC, 

Petterson  1895 

and  solid  and  liquid  hydrocarbons 
Water  evolves  hydrogen 

•o 

.Zirconium  
f  13oron 

zac 

B?C 

Moissan,   1896 

1894 

"          "      methane,  acetylene  and  ethylene 
Harder  than  corundum 

h 

1  Chromium  
1  Molybdenun... 

-{  Silicon 

Cr3Ca 
MoaC 

SiC 

41         1894 
1893 
1  Acheson,  im 

"          "    topaz 
11    corundum 

INot  dec 

hv* 

•*    Titanium  
Tungsten  ... 
(.Vanadium  

TiC 

wac 

VaC 

(  Moissan, 
Moissan,   1895 
1893 
1893 

Takes  fire  at  red  heat 
Harder  than  corundum 
'      quartz 

HEATING    EFFECTS.  133 

506.  Are  there  other  carbides  than  those  of  calcium  and  silicon 
that  are  or  can  be  made  in  the  electric  furnace? 

Quite  a  number  have  been  discovered,  as  shown  in  the  table : 

507.  How  is  calcium  carbide  made? 

Calcium  carbide  is  formed  by  the  union  of  carbon  and  calcium  at 
a  high  temperature.  The  calcium  is  obtained  from  common  line, 
which  is  calcium  oxide,  and  is  represented  chemically  by  the  abbre- 
viation CaO.  When  this  is  ground  fine  and  well  mixed  with 
powdered  coke  (carbon),  the  latter  unites  with  both  the  elements 
in  the  lime,  two  atoms  of  carbon  uniting  with  one  atom  of  calcium 
to  form  calcium  carbide  (CaCs),  the  other  atoms  of  carbon  uniting 
with  equal  numbers  of  atoms  of  oxygen  to  form  a  gas  known  as  car- 
bon monoxide  (CO).  The  carbon  monoxide  escapes  from  the  furnace 
and  unites  with  oxygen  of  the  air,  burning  with  a  blue  flame  and 
forming  carbon  dioxide  (CO),  which  is  more  familiarly  known  as 
carbonic  acid  gas.  The  reaction  is  expressed  by  the  chemical  equa- 
tion, 
CaO  (calcium  oxide)  +  3C  (carbon)  —  CaC«  (calcium  carbide)  + 

CO  (carbon  monoxide), 

which  is  read :  one  molecule  of  calcium  monoxide  unites  with  three 
molecules  of  carbon  to  form  one  molecule  of  calcium  carbide  and 
one  molecule  of  carbon  monoxide.  The  calcium  carbide  is  a  bluish 
gray  structureless  mass  easily  crumbled  to  powder.  It  unites 
greedily  with  water  forming  acetylene  gas  (C^H^)  and  slacked  lime 
(Ca(HO)2),  the  reaction  being  expressed  by  the  formula  :  CaC*  (cal- 
cium carbide)  +  2HzO  (water)  =  C2H2  (acetylene)  +  Ca(HO)« 
(calcium  hydroxide).  Acetylene  is  a  colorless  and  highly 
explosive  gas  with  very  disagreeable  smell,  burning  with  a  brilliant 
light,  but  apt  to  give  a  smoky  flame.  It  can  be  liquefied  at  a  pres- 
sure of  about  725  Ibs.-  per  square  inch  at  the  temperature  of  freezing 
water.  It  is  coming  into  extensive  use  as  an  illuminant. 

508.  How  is  carborundum  made? 

Carborundum  is  made  in  an  electric  furnace  from  a  mixture  of 
sand,  coke,  sawdust  and  salt.  The  real  action  is  between  the  sand, 
which  is  oxide  of  silicon  (SiO*)  often  called  silica,  and  the  carbon 
in  the  coke  and  sawdust.  The  sawdust  makes  the  mixture  porous 
to  facilitate  the  escape  of  gases,  and  the  salt  seems  to-  act  as  a  sort 
of  flux.  At  the  high  temperature  the  silica  is  dissociated,  its  silicon 
uniting  with  carbon  to  form  carbide  of  silicon  (SiC),  and  its  oxygen 
uniting  with  other  carbon  to  form  carbon  monoxide  (CO),  which 


134 


ELECTRICAL   CATECHISM. 


then  unites  with  more  oxygen  from  the  air  and  burns  into  carbonic 
acid  gas  (CO).     The  formulae  are: 

SIQ2  (silica)  +  30  (carbon)  =  SiC  (carbide  of  silicon)  +  2CO 
(carbon  monoxide)  ; 


FIG.  509.-CARBORUNDUM  FURNACE, 


HEATING   EFFECTS. 


135 


2CO  (carbon  monoxide)  +  O  (oxygen;  =  2CO  (carbonic  acid 
gas) .  The  carbide  of  silicon,  or  carborundum,  forms  in  small  flat,  thin 
crystals  of  beautiful  colors.  These  are  crushed  and  made  into 
various  shapes  for  abrasives,  doing  much  of  the  work  for  which 
emery  was  formerly  used. 

509.  In  what  sort  of  an  electric  furnace  is  carborundum  made? 

As  used  by  the  Carborundum  Company  at  Niagara  Falls,  the  fur- 
nace walls  are  of  brick,  being  about  16  ft.  long,  5  ft.  wide  and  5  ft. 
high,  the  ends  being  of  brick  about  2  ft.  thick.  In  the  center  of 
each  end  are  the  terminals,  each  consisting  of  six  carbon  rods  30 
ins.  long  and  3  ins.  in  diameter,  attached  to  an  iron  plate.  A  central 
core  between  the  two  sets  of  carbon  rods  is  made  of  coke  surrounded 
by  the  mixture  of  sand,  coke,  sawdust  and  salt,  making  a  cylinder 
about  21  ins.  in  diameter  and  14  ft.  long.  The  side  walls,  which  are 
taken  down  at  each  charge,  are  then  built  up  and  the  remaining 
space  is  rilled  with  the  mixture.  Current  of  something  more  than 
looo  amp.  at  100  volts  to  250  volts  is  then  sent  through  the  furnace 
for  about  twenty-four  hours.  This  furnace  works  on  the  incan- 
descent principle,  there  being  no  true  arc  present. 

510.  In  what  sort  of  a  furnace  is  calcium  carbide  made? 
Several  different  furnaces  are  employed,  nearly  all  of  them  being 

of  the  arc  type.  One  of  the  simplest  is  that  used  by  Borchers,  an 
eminent  German  chemist,  consisting  of  two  large  carbon  rods  which 
pass  through  limestone  or  similar  non-conductor.  These  rods  are 
separated  a  short  distance  and  are  connected  by  a  small  carbon  rod 
which  is  surrounded  by  the  material  (carbon  and  lime  for  calcium 
carbide)  to  be  acted  upon.  When  a  heavy  current  passes,  the  small 
carbon  rod  is  heated  to  incandescence  and  is  burned ;  an  arc  then 
follows  and  the  high  temperature  fuses  the  carbon  and  lime,  which 


FIG.   510.-ELECTRIC  ARC   FURNACES. 


136  ELECTRICAL  CATECHISM. 

then  unite  as  noted  in  No.  507.  The  furnaces  mostly  used  in  the 
United  States  have  two  carbons  arranged  one  above  the  other ;  the 
upper  one  is  movable  vertically  and  also  sometimes  horizontally. 
After  the  arc  is  struck,  the  upper  carbon  is  gradually  raised  as  the 
material  below  it  is  changed.  In  some  of  the  furnaces  the  arc  is 
maintained  at  65  volts  to  70  volts,  and  the  current  is  from  400  amp. 
to  1000  amp.  To  produce  one  ton  of  carbide  requires  about  200  hp- 
hours.  In  some  of  the  furnaces  one  of  the  carbons  rotates  slowly, 
so  as  to  keep  acting  on  new  material. 

511.  How  is  atmospheric  nitrogen  fixed  by  electrical  methods ? 
At  high  temperatures  nitrogen  combines  chemically  with  oxygen 

in  equal  parts  to  form  nitric  oxide  (NO).  In  the  Bradley-Love  joy 
process,  nitrification  was  accomplished  by  passing  air  through  a 
chamber  in  which  an  arc  was  broken  about  7000  times  a  second.  In 
the  Birkeland  process,  now  in  commercial  use,  air  is  passed  through 
an  alternating  current  arc  of  large  section.  The  resulting  nitric 
oxide  is  rapidly  removed  and  is  combined  with  water,  caustic  soda, 
etc.,  to  form  various  nitrates  and  nitrites  used  for  fertilizers,  ex- 
plosives, etc. 

512.  What  kind  of  current  is  used  in  electric  furnaces f 
Either  direct  or  alternating  current  may  be  used  in  incandescent 

and  in  some  of  the  arc  furnaces.  Only  the  direct  current  can  be 
used  in  furnaces  dependent  partly  or  wholly  upon  the  chemical  ef- 
fect of  the  current,  as  in  the  production  of  aluminum.  • 

513.  How  is  aluminum  produced? 

Aluminum  is  made  from  bauxite,  a  compound  of  aluminum, 
oxygen  and  water.  The  ore,  which  is  obtained  principally  from 
Georgia  and  Alabama,  is  treated  by  chemical  processes  to  remove 
impurities  and  reduce  it  to  aluminum  oxide  (AkOs),  commonly  called 
alumina.  Aluminum  is  sometimes  prepared  from  cryolite,  sulphate 
of  aluminum  and  from  clay,  but  in  America  it  is  made  principally 
from  bauxite.  The  alumina  is  dissolved  in  a  bath  of  melted  fluorides 
of  aluminum  with  sodium  or  potassium  and  calcium.  This  is  con- 
tained in  an  iron  pot  lined  with  carbon,  which  acts  as  the  negative  ter- 
minal, the  current  coming  in  through  cylinders  or  rods  in  the  upper 
part  of  the  pot.  Under  the  action  of  intense  heat,  combined  with 
electrolysis,  metallic  aluminum  is  deposited  in  the  bottom  of  the  pot, 
while  the  oxygen  liberated  from  the  alumina  goes  to  the  positive  ter- 
minal. About  5  hp-hours  are  required  to  reduce  i  Ib.  of  aluminum. 
The  production  of  aluminum  has  increased  from  83  Ibs.  in  1883  to 


HEATING   EFFECTS.  137 

550,000  Ibs.  in  1894,  and  8,000,000  Ibs.  in  1898,  more  than  half  of  the 
production  being  in  the  United  States.  The  price  of  aluminum  has 
fallen  from  $250.00  per  pound  in  1855  to  $0.25  in  1900. 

514.  What  is  the  difference  between  aluminum  and  aluminium f 
The  two  are  simply  different  forms  of  the  same  word,  the  former 

being-  somewhat  easier  and  shorter  to  pronounce.  On  account  of  the 
difficulty  in  distinguishing  between  aluminum  and  alumina  as  pro- 
nounced, the  men  around  the  reduction  works  use  the  longer  word 
aluminium  (pronounced  a-lu-min-i-um),  rather  than  aluminum  (pro- 
nounced (a-lu-mi-num). 

515.  Where  can  furtHer  accounts  of  electric  furnaces  be  found  f 
"  Electric    Furnaces    and   their    Industrial    Applications,"    by    J. 

Wright ;  "  Electric  Smelting  and  Refining,"  by  W.  Borchers  and 
W.  G.  McMillan ;  "  The  Electric  Furnace,"  by  Moissan ;  Transac- 
tions of  the  American  Electrochemical  Society;  Electrochemical 
and  Metallurgical  Industry.  Sections  on  "  Electrochemistry "  in 
"  Standard  Handbook  for  Electrical  Engineers "  and  in  Foster's 
"  Electrical  Engineer's  Pocket  Book." 


CHAPTER  V. 


BATTERIES   AND  ELECTROCHEMICAL 
ACTION. 

600.  What  is  the  relation  between  chemistry  and  electricity? 
When  a  current  passes  through  any  liquid  conductor,  the  liquid 

is  decomposed  or  otherwise  chemically  changed,  unless  it  is  a  chemi- 
cal element  such  as  mercury  or  a  melted  metal.  On  the  other  hand, 
when  two  conducting  substances  are  immersed  in  a  liquid  which 
dissolves  one  more  than  the  other,  an  electromotive  force  is  found 
to  exist  between  the  two.  The  latter  is  the  basis  of  all  batteries 
for  generating  electricity.  The  former  is  the  basis  of  the  storage 
battery,  and  of  many  applications  of  electricity  to  medicine,  to  chemi- 
cal manufacture,  to  electrical  measurement,  to  electro-plating,  and 
many  other  processes.  (See  Nos.  815  to  821  under  Instruments.) 
There  is  a  definite  relation  between  the  amount  of  electrical  energy 
absorbed  and  the  chemical  action  produced  in  each  case.  Chemical 
action  may  also  be  caused  indirectly  by  electricity,  as  in  the  electric 
furnace,  where  the  heating  effect  of  the  current  causes  very  high 
temperatures  which  loosen  the  bonds  and  allow  compounds  to  break 
up  so  that  their  elements  may  unite  in  new  combinations.  (See  Nos. 
503  to  513).  This  is  the  basis  of  electro-metallurgical  processes,  such 
as  the  manufacture  of  calcium  carbide  and  carborundum.  Chemical 
action,  like  heat,  may  thus  cause,  or  be  caused  by  electricity. 

601.  What  is  a  chemical  cell  or  battery  for  generating  electricity? 
It  consists  of  two  pieces  of  conducting  substances  in  a  liquid  that 

acts  chemically  upon  one  more  than  upon  the  other.  In  primary  cells 
one  of  the  substances  is  generally  a  rod  or  plate  of  zinc,  a  metal  that 
is  cheap  and  also  easily  acted  upon  by  solutions  of  common  salts. 
The  other  substance  is  generally  carbon,  copper  or  iron.  The  solu- 
tion is  generally  sal  ammoniac,  or  copper  sulphate,  or  caustic  potash. 
Sometimes  acids  are  used.  The  conductor  that  is  least  eaten  by  the 
liquid  becomes  positively  charged  and  the  other  negatively  charged. 
In  secondary  cells  the  conductors  generally  consist  of  lead  and  its 
compounds,  and  the  liquid  is  dilute  sulphuric  acid. 


ELECTROCHEMISTRY.  139 

602.  What  is  the  difference  between  a  cell  and  a  battery? 

The  word  "battery"  refers  strictly  to  a  number  of  cells  coupled 
together.  It  is  quite  common,  although  inaccurate,  to  use  the  word 
when  referring  to  a  single  cell.  The  word  "cell"  often  refers  to  the 
containing  vessel. 

603.  What  is  a  voltaic  or  galvanic  battery? 

These  are  simply  other  names  for  chemical  batteries.  Volta  and 
Galvani  are  the  two  men  who  divide  honors  for  discovering  the  prin- 
ciple and  inventing  the  battery. 

604.  For  what  purposes  are  batteries  used? 

Where  small  currents  are  used,  as  for  electric  door  bells,  telegraph 
lines,  telephone  transmitters,  medical  purposes  and  for  very  small 
motors,  primary  batteries  are  usually  the  most  convenient  source  of 
current.  Where  larger  quantities  of  electricity  are  required,  as  for 
operating  electric  automobiles,  telephone  exchanges  and  for  use  in 
connection  with  central  stations  for  electric  light  and  power,  the 
storage  battery  has  a  useful  field. 

605.  What  is  the  difference  between  a  primary  and  a  secondary 
battery? 

When  a  primary  battery  is  exhausted,  it  is  necessary,  as  a  rule, 
to  throw  away  the  electrolyte  and  supply  a  new  one,  also  frequently 
renewing  the  electrodes.  Storage  batteries  are  recharged  by  send- 
ing a  current  through  them  in  the  opposite  direction  from  that  which 
the  battery  delivers,  the  same  electrolyte  and  electrodes  being  used 
over  and  over  again.  In  the  secondary,  or  storage,  battery  there  is 
no  storing  of  electricity  as  such.  The  charging  current  reverses  cer- 
tain chemical  actions  (see  Nos.  635  to  638),  and  the  energy  that  is 
stored  is  chemical  rather  than  electrical.  The  only  case  where  elec- 
tricity is  stored  as  such  is  in  the  condenser  or  Leyden  jar.  (See  Nos. 
in  to  123.) 

606.  What  is  an  electrolyte? 

Liquids  may  be  classified  into  three  divisions :  those  that  do  not 
conduct  electricity  at  all,  such  as  oils ;  those  that  conduct  electricity 
without  any  chemical  effects,  such  as  mercury  and  other  metals  in 
liquid  form  that  act  like  solid  metals ;  and  those  that  are  decomposed 
by  the  passage  of  current.  Liquids  of  the  third  class  are  called 
electrolytes. 

607.  What  is  an  electrode? 

Electrode  is  a  name  for  the  solid  conductors  which  lead  the 
current  to  and  from  the  electrolyte.  The  anode,  sometimes  thought 


140  ELECTRICAL   CATECHISM. 

of  as  "in-ode,"  is  the  plate  at  which  the  current  enters  .the  liquid, 
being-  the  positive  terminal.  The  cathode  is  the  plate  at  which  the 
current  leaves,  being  the  negative  terminal. 


- 

__  _____      ~ 

:E-_E 

~ 

ELECTROLYTE 

~ 

T2 

_  ~zn_,~z_z=r  i 

/ 

*NOD£                     CATHODE 

FIG.  607.— ELECTROLYTIC  CELL. 

608.  What  is  meant  by  the  electro-chemical  series? 

The  difference  of  potential  between  the  electrodes  varies  with  dif- 
ferent substances.  For  example,  zinc  and  copper  give  several  times 
as  much  E.M.F.  as  zinc  and  iron,  while  zinc  and  carbon  give  still 
more.  It  is  found  also  that  if  copper  and  carbon  be  placed  in  dilute 
acid,  they  form  a  battery ;  current  will  flow  from  the  carbon  through 
the  wire  to  the  copper,  and  the  carbon  is  said  to  be  electro -positive 
to  the  copper.  On  the  other  hand,  if  copper  and  zinc  are  used,  the 
copper  is  the  more  positive  and  current  flows,  through  the  wire  from 
copper  to  zinc.  The  various  elements  may  be  arranged  in  such  an 
order  that  each  is  electro-positive  to  those  following  and  electro- 
negative to  those  preceding.  The  common  order  is  as  follows : 
Oxygen,  sulphur,  selenium,  nitrogen,  fluorine,  chlorine,  bromine, 
iodine,  phosphorus,  arsenic,  chromium,  vanadium,  molybdenum, 
tungsten,  boron,  carbon,  antimony,  tellurium,  titanium,  silicon,  hy- 
drogen, gold,  platinum,  palladium,  mercury,  silver,  copper,  bismuth, 
tin,  lead,  aluminum,  cavlmium,  cobalt,  nickel,  iron,  zinc,  manganese, 
uranium,  magnesium,  cacium,  strontium,  barium,  lithium,  sodium, 
potassium.  The  order  of  these  elements  varies  to  some  extent  with 
the  electrolyte. 

609.  How  can  one  tell  ivhich  is  the  positive  and  which  the  nega- 
tive terminal  of  a  battery? 

In  most  cells  the  electrodes  are  of  different  metals,  and  only  one  is 
dissolved  into  the  solution.  The  metal  which  is  subject  to  the  more 


ELECTROCHEMISTRY.  141 

vigorous  chemical  action  may  be  considered  as  the  fuel  that  supplies 
the  energy,  and  it  is  called  the  positive  plate.  But  it  is  always  found 
that  the  metal  which  is  the  more  active  chemically  is  electro-negative 
to  the  other.  So  it  comes  about  that  the  positive  plate  constitutes  the 
negative  terminal  of  the  cell,  and  the  negative  plate  is  the  positive 
terminal  of  the  cell.  Thus,  zinc  is  generally  used  for  the  positive 
plate  in  cells,  and  the  zinc  is  generally  the  negative  terminal  of  the 
cell.  The  rule  is  easily  remembered  that  the  substance  not  destroyed 
is  the  positive  terminal  of  the  cell. 

610.     How  many  kinds  of  batteries  are  there? 

Batteries  are  classified  as  primary  and  secondary,  as  noted  already. 
Primary  batteries  are  further  divided  into  "open  circuit"  and  "closed 
circuit"  batteries.  They  may  also  be  divided  into  three  classes,  ac- 
cordingly as  the  depolarizer  is  mechanical,  chemical  or  electro-chemi- 
cal. 

6n.     What  is  meant  by  an  open  circuit  battery? 

An  open  circuit  battery  is  one  that  is  suited  for  only  intermittent 
work,  such  as  operating  door  bells  and  similar  purposes.  Such  cells 
polarize  quickly  when  in  operation,  so  that  their  resistance  rapidly  in- 
creases and  diminishes  the  current  after  a  few  minutes.  Such  cells 
recover  after  a  time  as  the  polarization  gradually  disappears. 

612.  What  is  meant  by  the  polarization  of  a  battery? 

When  the  cell  is  generating  current,  the  metal  dissolved  from  the 
positive  plate  combines  with  the  electrolyte,  and  forms  with  it  a  new 
compound.  Usually  the  dissolved  metal  displaces  hydrogen  gas, 
which  appears  in  the  form  of  bubbles  on  the  surface  of  the  negative 
plate.  These  bubbles  diminish  the  amount  of  surface  of  plate  in  con- 
tact with  the  liquid,  and  so  increase  the  resistance  of  the  cell,  and 
thus  reduce  the  current.  The  hydrogen  bubbles  are  strongly  electro- 
positive, and  so  set  up  an  opposing  E.M.F.  which  further  reduces 
the  current. 

613.  How  is  the  polarization  removed? 

In  some  cells  the  gas  bubbles  rise  to  the  surface  and  escape  to  the 
air,  the  surface  of  the  negative  plate  being  made  rough,  so  as  to  offer 
many  fine  points  where  the  gas  may  collect,  and  escape.  In  other 
cases  a  "depolarizer"  is  provided  to  unite  chemically  with  the  gas,  and 
form  a  substance  that  will  dissolve  in  the  liquid.  For  this  purpose 
black  oxide  of  manganese,  oxide  of  copper,  red  lead,  peroxide  of 
lead,  sulphur,  bromine,  chlorine,  nitric  acid  and  solutions  of  chromic 
acid,  of  bichromate  of  soda,  of  bichromate  of  potash,  of  nitrate  of 


142 


ELECTRICAL   CATECHISM. 


potash  and  of  ferric  chloride  are  used.  Some  of  these  attack  zinc 
even  when  the  circuit  is  not  closed,  and  must  be  used  with  care.  The 
solid  depolarizers  generally  work  only  slowly,  and  ctre  used  in  cells 
for  intermittent  work  on  circuits  that  are  generally  open.  Liquid 
depolarizers  generally  work  rapidly,  and  the  cell  may  be  used  con- 
tinuously with  but  little  diminution  of  the  E.M.F. 

614.     What  are  examples  of  open  circuit  batteries? 

The  cells  using  carbon  and  zinc  for  electrodes  usually  polarize  in 
a  short  time.  The  electrolyte  is  usually  a  solution  of  sal  ammoniac 
(ammonium  chloride)  in  water,  although  other  salts  are  sometimes 
used.  The  larger  the  amount  of  surface  on  the  carbon  electrode,  the 
longer  the  cell  will  deliver  current  without  troublesome  polarization. 
The  larger  the  surface  of  carbon  and  of  zinc,  and  the  closer  they  are 
together,  the  less  the  internal  resistance  of  the  cell  and  the  more  cur- 
rent it  will  deliver.  Excellent  forms  of  sal  ammoniac  cells  are 


FIG.  614.— SAL  AMMONIAC  CELLS. 

shown  in  the  figures.  Some  have  a  depolarizing  substance  in  con- 
nection with  the  carbon,  while  others  depend  on  the  large  surface 
and  the  slow  removal  of  the  polarizing  gases. 

615.     Explain  the  action  of  the  sal  ammoniac  cell? 

This  cell  consists  of  carbon  and  zinc  electrodes,  immersed  in  a  so- 
lution of  sal  ammoniac  (NHiCl),  technically  known  as  ammonium 
chloride.  The  solution  Has  no  effect  on  either  electrode  when  the 
circuit  is  open.  But  when  the  circuit  is  closed,  the  zinc  is  dissolved, 
and  breaks  up  the  sal  ammoniac  to  form  zinc  chloride  (ZnCIO, 


ELECTROCHEMISTRY.  143 

ammonia  gas  (NHs),  and  hydrogen  gas  (Ha).  The  ammonia  im- 
mediately dissolves  in  the  water  of  the  solution,  and  forms  ammonia 
water  (NH^OH),  while  the  hydrogen  collects  on  the  carbon  elec- 
trode. In  many  cells,  such  as  the  Law,  Diamond  and  Laclede  cells, 
the  hydrogen  simply  collects  on  the  carbon  during  the  action  of  the 
cell,  and  then  gradually  rises  to  the  surface,  and  escapes  to  the  air 
while  the  cell  is  resting.  The  larger  the  surface  of  the  carbon,  the 
longer  the  cell  will  work  without  rest.  Other  forms  of  sal  ammoniac 
cell  have  a  chemical  depolarizer,  and  are  known  by  the  general  name 
Leclanche  cell.  Sal  ammoniac  cells  give  from  1.3  to  1.7  volts  when 
new. 

6 1 6.  Explain  the  action  of  the  Leclanche  cell? 

In  the  Leclanche  cell  the  carbon  is  placed  in  contact  with  some 
solid  substance  which  will  unite  chemically  with  the  hydrogen,  and 
so  remove  the  polarization.  In  the  original  Leclanche  cell  the  carbon 
was  placed  in  a  porous  cup,  which  was  then  filled  with  manganese 
dioxide  (MnO),  which  readily  gave  up  part  of  its  oxygen  to  form 
water  (HXD)  with  the  hydrogen,  and  became  reduced  to  a  lower 
oxide  (MmO).  The  manganese  was  mixed  with  fragments  of  car- 
bon to  increase  the  conductivity,  and  to  hold  more  gas.  The  porous 
cup  was  found  to  increase  the  internal  resistance  of  the  cell,  and  the 
manganese  dioxide  was  then  mixed  with  carbon  and  cement  to  form 
blocks,  which  were  then  held  against  the  carbon  plate  .by  means  of 
rubber  bands.  The  next  step  was  to  make  the  carbon  in  the  form  of 
a  hollow  cylinder,  and  place  the  depolarizer  inside,  as  in  the  Samson 
(see  Fig.  614)  and  the  Hayden  cells.  In  some  cells  the  depolarizer  is 
mixed  with  the  carbon,  and  both  are  molded  and  baked  .in to  a  solid 
mass ;  such  cells  do  excellent  work  at  first,  but  are  apt  to  wear  out 
quicker  than  those  in  which  the  depolarizer  can  be  renewed.  Many 
millions  of  sal  ammoniac  cells  are  used  for  operating  bells  and  tele- 
phones. 

617.  How  strong  should  the  solution  be  in  a  sal  ammoniac  cell? 
The  directions  vary  with  different  cells,  but  about  five  ounces  of 

the  dry  salt  to  one  quart  of  water  is  an  average  solution.  If  the 
solution  is  too  strong,  a  double  salt  of  the  chlorides  of  zinc  and 
ammonium  is  liable  to  crystallize  out  and  deposit  on  the  zinc,  thus  in- 
creasing the  internal  resistance  of  the  cell  and  also  lowering  its 
E.M.F. 


144  ELECTRICAL  CATECHISM. 

618.  Why  do  the  zincs  in  sal  ammoniac  cells  usually  eat  through 
at  the  top  faster  than  at  the  bottom? 

At  the  surface  of  the  liquid  there  is  more  or  less  oxidation,  but  this 
does  not  explain  the  fact  that  the  zinc  becomes  gradually  thinner 
toward  the  top.  There  is  generally  more  or  less  of  the  double 
chloride  of  zinc  and  ammonium  present  in  every  sal  ammoniac  cell. 
As  this  is  heavier  than  the  mixed  solution  of  zinc  chloride  and  of 
ammonium  chloride,  it  settles  toward  the  bottom  of  the  cell.  There 
is  then  a  local  action  which  tends  to  dissolve  the  zinc  at  the  top  and 
to  deposit  it  at  the  bottom,  for  zinc  in  a  solution  of  zinc  chloride  is 
positive  to  zinc  in  a  solution  of  the  double  salt ;  the  zinc  and  the 
liquid  thus  become  a  short-circuited  cell. 

619.  Why  are  zincs  better  when  amalgamated? 
Commercial  zinc  is  not  chemically  pure,  but  contains  impurities 

such  as  bits  of  iron,  carbon  and  other  substances,  When  the  zinc  is 
immersed  in  any  liquid  which  attacks  the  zinc  more  than  the  impuri- 
ties, an  E.M.F.  is  set  up;  as  the  two  substances  are  connected 
through  the  liquid  and  also  through  the  metal,  local  currents  are  set 
up  which  eat  away  the  zinc  until  the  foreign  substance  is  set  free  and 
falls  away.  When  the  zinc  is  amalgamated ,  that  is,  coated  or  alloyed 
with  mercury,  the  mercury  seems  to  cover  up  the  impurities  and  to 
bring  only  the  pure  zinc  to  the  surface.  The  smooth  surface  seems 
to  hold  a  film  of  hydrogen  when  the  cell  is  not  at  work,  and  this 
film  seems  to  protect  the  zinc  from  attack  when  acid  is  present,  and 
to  protect  it  from  local  action  at  all  times. 

620.  How  are  zincs  amalgamated? 

A  common  way  is  to  dip  the  zinc  in  dilute  sulphuric  acid,  which 
cleans  it  thoroughly,  and  then  to  rub  mercury  over  it  with  a  swab 
until  the  zinc  is  uniformly  bright  all  over.  The  swab  may  be  simply 
a  piece  of  cloth  wrapped  around  a  wooden  stick.  In  some  cases 
the  zinc  and  mercury  are  cast  into  a  sort  of  alloy  or  amalgam,  the 
zinc  being  melted  first  and  the  mercury  added. 

621.  What  is  the  silver  chloride  cell? 

This  cell  uses  a  zinc  rod  for  positive  electrode.  The  negative  con- 
sists of  a  silver  rod  surrounded  by  silver  chloride,  which  is  melted 
into  a  cylinder  upon  the  rod.  The  electrolyte  is  a  solution  of  sal 
ammoniac.  When  the  cell  generates  current,  the  zinc  displaces  the 
chlorine  in  the  sal  ammoniac,  and  the  ammonium  thus  set  free  dis- 
places the  chlorine  in  the  silver  chloride,  leaving  metallic  silver  de- 
posited on  the  silver  electrode.  No  gas  is  set  free  unless  the  cell  is 
worked  too  hard.  This  cell  gives  about  i.i  volts  and,  being  small 


ELECTROCHEMISTRY.  145 

and  easily  portable,  is  much  used  for  testing  purposes,  and  to  some 
extent  for  medical  purposes. 

622.  What  are  dry  cells? 

Dry  cells  are  generally  modifications  of  sal  ammoniac  cells,  in 
which  the  water  is  largely  replaced  by  some  gelatinous  substance  of 
more  or  less  secret  composition.  The  original  Gassner  dry  cell  used 
a  paste  made  of  I  part  oxide  of  zinc,  I  of  sal  ammoniac,  3  of  plaster, 
I  of  zinc  chloride  and  2  of  water,  all  by  weight.  Dry  cells  are  very 
convenient  for  portable  use  and  in  the  hands  of  unskilled  persons, 
but  their  useful  life  is  generally  much  shorter  than  that  of  cells  using 
solution  of  sal  ammoniac  in  water. 

623.  Why  is  it  necessary  to  remove  the  zinc  from  some  forms 
of  open  circuit  battery  when  not  in  use? 

In  some  cells  which  use  permanganate  of  potash  or  sulphuric  acid 
in  order  to  secure  a  high  E.M.F.,  the  electrolyte  acts  upon 
the  zinc  by  ordinary  chemical  action,  even  when  not  delivering 
current ;  such  cells  are  therefore  provided  with  means  for  removing 


FIG.  623.— GRENET  CELL. 

the  zinc  when  not  in  use.  A  "Grenet"  cell  of  this  type,  illustrated  in 
the  figure,  shows  plainly  the  rod  and  clamp  for  raising  the  zinc  plate 
in  the  center  between  the  positives. 

624.     Explain  the  action  of  the  bichromate  cell 

The  bichromate  cell  takes  various  forms,  being  commonly  ar- 
ranged so  that  the  zinc  or  both  elements  may  be  lifted  easily  from 
the  liquid.    They  are  often  called  "plunge  batteries,"  since  the  ele 
ments  must  be  plunged  into  the  solution  before  the  current  is  gen- 
erated.    The  liquid  contains  sulphuric  acid  (H»  SO*)  and  chromic 


146  ELECTRICAL   CATECHISM. 

acid  (CrOs).  When  the  cell  is  in  operation,  the  zinc  is  dissolved 
by  the  sulphuric  acid,  forming  zinc  sulphate  (ZnSO*),  the  hydrogen 
set  free  unites  with  the  oxygen  in  the  chromic  acid  to  form  water, 
and  the  chromium  thus  left  unites  with  part  of  the  sulphuric  acid  to 
form  chromium  sulphate  (Cr2(SO*)s).  In  this  form  of  cell,  the 
chromic  acid  is  generally  made  when  the  cell  is  first  set  up,  by  pour- 
ing sulphuric  acid  (78.5  cu.  cm.)  into  a  solution  of  77.5  grams  of 
bichromate  of  potassium  (Ka  Cn  O)  in  750  cu.  cm.  of  water;  the 
acid  and  bichromate  unite  to  form  chromic  acid  and  potassium  sul- 
phate (Kf  SO*)-  The  latter  unites  with  the  chromic  sulphate  formed 
during  the  operation  of  the  battery  and  forms  a  compact  mass  of 
crystals  of  chrome  alum  (Ka  Cn  (SO*)*).  Sodium  bichromate  is 
often  used  instead  of  potassium  bichromate  in  the  proportion  of  200 
grams  of  sodium  bichromate  to  1000  cu.  cm.  of  water  and  150  cu.  cm. 
of  strong  sulphuric  acid,  more  acid  being  added  as  the  battery  be- 
comes exhausted.  Sometimes  the  chromic  acid  is  used  directly,  being 


FIG.  624.— PARTZ  CELL. 


used  in  the  proportion  of  150  grams  to  1000  cu.  cm.  of  water  and  150 
cu.  cm.  of  strong  sulphuric  acid.  Any  form  of  bichromate  cell  gives 
2  volts  when  fresh.  The  Partz  bichromate  cell,  which  looks  some- 
what like  the  gravity  cell  (see  No.  626),  has  a  carbon  element  at 


ELECTROCHEMISTRY.  147 

the  bottom,  a  zinc  element  near  the  top,  and  a  funnel  through  which 
fresh  chemical  salt  is  introduced  as  needed. 

625.  Explain  the  Fuller  cell. 

This  cell,  which  is  used  to  a  large  extent  by  telephone  companies, 
is  a  modification  of  the  bichromate  cell,  having  an  amalgamated  zinc 
in  a  porous  cup  filled  with  a  solution  of  common  salt  in  water.  1  he 
carbon  is  placed  in  the  outside  jar  with  a  solution  made  of  6  ounces 
of  sodium  bichromate,  17  ounces  of  sulphuric  acid  and  56  ounces  of 
soft  water.  A  little  mercury,  placed  in  the  bottom  of  the  porous 
cup,  keeps  the  zinc  well  amalgamated.  There  is  said  to  be  little  local 
action  and  the  cell  works  a  month  or  more  without  attention. 

626.  What  are  closed  circuit  batteries? 

In  closed  circuit  cells  it  is  usually  necessary  to  keep  the  circuit 
closed  all  of  the  time  to  prevent  injurious  action  in  the  cell,  as  it  is 
harmed  more  by  standing  idle  than  by  use.  The  best  known  ex- 
ample of  a  closed  circuit  cell  is  the  gravity  or  "crowfoot"  cell,-  used 
so  largely  for  telegraph  and  fire  alarm  circuits.  The  zinc  electrode 
is  commonly  cast  in  the  form  of  a  foot,  which  gives  it  the  common 


FIG.  626.-CROWFOOT  CELL. 

name.  The  copper  positive  plate  usually  consists  of  several  sheets 
of  copper  riveted  together  and  to  an  insulated  wire,  which  forms  the 
terminal.  Crystals  of  copper  sulphate,  often  called  "blue  vitriol"  or 
"blue  stone,"  are  placed  in  the  bottom  and  the  cell  is  filled  with  water. 
As  the  cell  stands,  the  copper  sulphate  dissolves  in  the  water  and  the 
solution  rises  higher  and  higher  until  it  reaches  the  zinc,  unless  the 
circuit  is  closed.  In  this  cell  the  zinc  is  dissolved,  forming  zinc 
sulphate  at  the  top  of  the  cell,  while  copper  is  deposited  from  the 
blue  solution  upon  the  copper  plates  at  the  bottom.  So  long  as  the 
ceil  is  working,  there  is  a  more  or  less  distinct  division  between  the 
two  solutions,  but  if  the  cell  is  left  on  open  circuit,  the  copper  so- 


148 


ELECTRICAL   CATECHISM. 


lution  rises  until  it  comes  into  contact  with  the  zinc,  where  the  cop- 
per deposits  and  a  local  action  rapidly  eats  up  the  zinc  and  exhausts 
the  solution.  As  the  copper  solution  is  heavier  than  the  zinc,  it 
tends  to  stay  at  the  bottom  of  the  cell  and  gravity  keeps  the  zinc 
and  copper  solutions  apart  when  the  cell  is  working.  This  gives  it 
the  name  of  "gravity  cell." 

627.  Are  any  cells  suitable  for  both  intermittent  and  continuous 
work? 

A  storage  cell  is  suitable  for  both  kinds  of  work,  provided  not  too 
long  a  period  of  idleness  is  imposed  upon  it,  in  which  case  the  cell 
gets  out  of  order  and  loses  its  charge.  There  are  a  few  primary  cells 
that  are  well  adapted  for  continuous  work,  and  also  will  stand  idle 
without  any  deleterious  action.  One  of  the  best  known  of  these  is 
the  Edison-Lalande  cell. 

628.  Explain  the  action  of  the  Edison-Lalande  cell. 

In  this  cell  the  positive  electrode  is  one  or  more  plates  of  zinc  and 
the  negative  electrode  consists  of  a  slab  of  copper  oxide  held  in  a 


FIG.  628A.-GORDON  CELL. 


FIG.  628B.-EDISON  LALANDE  CELL. 

frame  of  copper.  The  electrolyte  is  a  strong  solution  of  caustic 
soda  (NaOH)  or  caustic  potash  (KOH).  When  the  cell  is  work- 
ing the  zinc  dissolves,  forming  sodium  (or  potassium)  zincate 


ELECTROCHEMISTRY.  149 

(Naz  ZnOz),  and  displacing  the  hydrogen  which  moves  with  the 
current  to  the  negative  plate,  where  it  unites  with  the  copper  oxide 
to  form  water  (hydrogen  dioxide,  HzO),  and  metallic  copper.  The 
solution  is  always  covered  with  a  layer  of  heavy  paraffine  oil,  both 
to  prevent  evaporation  and  also  to  prevent  the  carbonic  acid  gas  of 
the  air  from  acting  on  the  solution.  When  the  oil  is  not  used  the 
life  of  the  cell  is  reduced  to  about  one-third.  The  Edison-Lalande 
cell  gives  about  0.7  volts.  The  Gordon  cell  is  similar  to  the  Edison- 
Lalande,  except  that  the  zinc  is  made  into  a  cylinder  surrounding 
the  inactive  element,  which  consists  of  a  perforated  tin  cylinder 
rilled  with  oxide  of  copper. 

629.     Give  an  example  of  a  cell  with  mechanical  depolarizer. 

The  Smee  cell,  formerly  used  to  a  considerable  extent,  is  a  good 
example.  This  had  plates  of  zinc  and  of  platinized  silver  immersed 
in  dilute  sulphuric  acid.  The  platinized  silver  had  a  rough  surface 
which  facilitated  the  collection  of  the  hydrogen  into  bubbles  large 
enough  to  rise  easily  to  the  surface  and  so  escape  to  the  air.  Carbon 
was  found  to  be  cheaper  than  platinized  silver  and  also  to  give  a 
higher  E.M.F.  The  action  of  the  cell  is  explained  by  saying  that  the 
zinc  dissolves  in  the  sulphuric  acid  (Hz  SO*),  forming  zinc  sulphate 
(ZnSO*)  and  thus  displacing  the  hydrogen  (Hz)  from  the  acid, 
which  then  collects  in  bubbles  on  the  silver  or  carbon  plate.  When 
the  action  of  the  cell  becomes  weak  from  the  collection  of  hydrogen 
bubbles,  they  can  be  removed  by  stirring  the  liquid  or  by  lifting  the 
plates  from  the  cell.  This  cell  gives  I  volt  when  fresh.  The  Smee 
cell  was  formerly  used  to  a  considerable  extent  for  electroplating 
and  for  similar  purposes  requiring  a  strong  current.  A  more  com- 
mon example  of  cell  with  mechanical  depolarization  is  the  sal  am- 
moniac cell  (see  No.  615)  used  for  ringing  bells  and  similar  inter- 
mittent work. 

630.  What  kind  of  cells  use  liquid  depolarizers? 

These  may  be  divided  into  two  classes,  those  with  single  liquids 
and  those  with  two.  The  former  class  is  represented  by  the  bichro- 
mate cells  (see  No.  624),  the  latter  by  the  gravity  (see  No.  626), 
Daniell  or  Bunsen. 

631.  Explain  the  action  of  the  Bunsen  cell. 

The  Bunsen  cell  contains  a  carbon  plate  in  a  cup  of  porous  earthen- 
ware filled  with  strong  nitric  acid  (HNO).  The  zinc  is  placed  in  a 
jar  outside  the  porous  cup  and  rilled  with  dilute  sulphuric  acid 
(H2  SO)  made  of  about  one  part  of  acid  to  twelve  of  water.  A 
number  of  molecules  of  the  sulphuric  acid,  ILSO*,  seem  to  be 


150  ELECTRICAL  CATECHISM. 

broken  up  into  free  hydrogen  ions,  H,  and  sulphions,  SO*  (see  No. 
644).  When  the  cell  is  in  operation,  the  sulphions  appear  at  the 
zinc  anode  and  form  zinc  sulphate,  ZnSO*,  while  the  hydrogen  ions 
appear  at  the  carbon  cathode,  where  they  take  part  of  the  oxygen 
from  the  nitric  acid  to  form  water  (IrbO)  and  leave  nitrous  acid 
(HNO),  or  hyponitrous  acid  (HNO)  ;  or,  the  hydrogen  may  even 
reduce  the  latter  acid  to  nitric  oxide  (NO),  which  escapes  as  a  very 
corrosive  and  objectionable  gas.  This  cell  gives  about  1.8  volts. 

632.  Explain  the  Daniell  cell 

The  Daniel  cell,  formerly  used  as  a  standard  of  E.M.F.,  is  some- 
what similar  to  the  Bunsen,  in  having  one  electrode  within  a  porous 
jar,  and  in  having  two  liquids.  A  common  form  has  a  zinc  rod  or 
plate  within  the  porous  jar,  which  is  filled  with  zinc  sulphate  dis- 
solved in  water  or  with  dilute  sulphuric  acid.  The  outer  vessel  con- 
tains a  copper  electrode  and  is  filled  with  a  solution  of  copper  sul- 
phate (CuSO*),  often  called  "blue  vitriol"  or  "blue  stone."  When 
the  cell  is  delivering  current,  part  of  the  zinc  sulphate  breaks  up 
into  zinc  ions  and  sulphions ;  the  zinc  ions  displace  copper  ions  from 
the  copper  sulphate  solution  and  the  displaced  copper  ions  appear 
at  the  copper  electrode  and  are  deposited  as  metallic  copper;  the 
sulphions  (see  Nos.  631  and  644)  appear  at  the  zinc  anode  and 
form  zinc  sulphate.  If  sulphuric  acid  were  used  without  the 
copper  sulphate,  the  zinc  in  uniting  with  the  acid  would  displace 
the  hydrogen,  which  would  then  tend  to  collect  at  the  copper.  In- 
stead of  collecting  in  bubbles,  however,  the  hydrogen  displaces  the 
copper  from  the  copper  sulphate,  which  thus  becomes  a  sort  of  elec- 
tro-chemical depolarizer. 

633.  What  governs  the  amount  of  current  delivered  by  a  battery 
of  one  or  more  cells? 

The  current  from  a  battery,  like  that  from  any  other  source,  is 
governed  by  Ohm's  law.  (See  Nos.  315  to  329).  The  current 
(measured  in  amperes)  equals  the  E.M.F.  (measured  in  volts)  di- 
vided by  the  total  resistance  of  the  circuit  (measured  in  ohms). 
This  resistance  includes  the  resistance  in  the  external  circuit,  and 
also  the  internal  resistance  of  the  cells.  If  the  resistance  of  the  ex- 
ternal circuit  is  high,  as  is  the  case  with  telegraph  lines,  the  internal 
resistance  of  the  cells  is  but  a  small  part  of  the  whole,  and  the  cells 
are  connected  in  series  to  give  high  E.M.F.  If  the  external  circuit  is 
of  low  resistance,  the  resistance  of  a  single  cell  may  be  high  as  com- 
pared with  that  of  the  external  circuit.  The  internal  resistance  of  a 
•cell  is  less  as  the  electrodes  are  larger  and  closer  together ;  by  con- 


ELECTROCHEMISTRY.  151 

necting  together  similar  electrodes  of  several  cells,  they  all  act  as  one 
large  cell  having  much  less  resistance  than  any  one,  the  combined  re- 
sistance of  several  cells  of  similar  size  and  construction  being  equal 
to  that  of  one  divided  by  the  number  of  cells  connected  in  multiple. 
Thus,  if  the  resistance  of  one  cell  is  I  ohm,  the  resistance  of  five  cells 
coupled  in  multiple  is  one-fifth  of  an  ohm.  Cases  often  arise  where 
it  is  best  to  make  a  combination  of  series  and  parallel,  connecting 
the  cells  in  several  sets  having  an  equal  number  of  cells  in  series  and 
then  connecting  these  sets  in  multiple.  The  greatest  output  is  ob- 
tained when  the  resistance  of  the  cells  equals  that  of  the  Line,  but 
this  is  not  the  most  economical  arrangement,  since  half  of  the  energy 
is  then  lost  in  the  cells.  For  the  highest  economy,  the  internal  resist- 
ance of  the  cells  should  be  as  low  as  practicable  in  comparison  with 
the  resistance  of  the  external  circuit. 

634.  What  is  a  gas  battery? 

The  simplest  one  is  that  of  Grove,  who  arranged  platinum  elec- 
trodes so  that  each  dipped  into  acidulated  water  and  also  was  in  con- 
tact with  a  gas.  When  one  electrode  was  in  hydrogen  and  the  other 
in  oxygen,  he  obtained  nearly  I  volt.  Many  attempts  have  been 
made  to  develop  a  commercial  battery  from  this,  but  none  have  been 
very  successful.  These  cells  give  a  high  theoretical  efficiency  and 
may  some  day  prove  to  be  the  basis  of  a  more  economical  source  of 
electricity  than  dynamos  driven  by  steam  or  water  power,  but  up 
to  the  present  time  they  are  simply  a  hope. 

635.  What  is  a  storage  battery? 

A  storage  battery  is  a  battery  which,  when  exhausted,  can  be  re- 
charged by  sending  current  through  it  from  another  source.  The 
earliest  note  of  this  was  a  discovery  by  Gautherot,  who  found,  in 
1 80 1,  that  silver  or  platinum  wires  used  for  decomposing  water  by 
the  passage  of  an  electric  current  would  send  a  current  in  the  reverse 
direction  when  the  battery  was  removed.  This  was  studied  further 
by  Ritter,  De  la  Rive  and  Grove,  who  developed  the  gas  battery.  The 
modern  storage  battery  is  developed  from  the  work  of  other  experi- 
menters with  lead  electrodes.  Faraday,  in  1834,  found  that  lead 
peroxide  (PbO*)  deposited  by  acetate  of  lead  (Pb^HsOOO 
would  give  a  current  in  the  opposite  direction  from  that  originally 
causing  the  decomposition.  In  1854,  Sinsteden  used  plates  of  lead, 
silver  and  nickel  in  a  voltameter  and  obtained  reverse  currents  strong 
enough  to  heat  a  wire  to  incandescence.  In  1860,  Plante  sent  cur- 
rent through  two  sheets  of  lead,  which  were  separated  by  canvas  and 
immersed  in  acid,  and  he  obtained  a  much  more  powerful  and  dura- 


152 


ELECTRICAL   CATECHISM. 


ble  secondary  current  than  had  Faraday  or  Sinsteden,  and  he  is 
generally  looked  upon  as  the  discoverer  or  inventor  of  the  storage 
battery.  Plante  found  that  the  capacity  of  his  lead  secondary  battery 
increased  with  use,  finding  that  the  surface  of  the  lead  was  changed 
into  sulphates  and  oxides  and  that  these  coatings  became  deeper  and 
deeper  with  use.  In  1881,  Faure  found  that  the  oxide  might  be  ap- 
plied mechanically  more  cheaply  than  by  forming  from  the  plate,  and 
his  pasted  cell  came  into  extensive  use. 

636.     How  are  storage  batteries  made  at  present? 

For  a  number  of  years,  the  Faure  process  was  used  more  gen- 
erally, but  at  present  it  seems  to  be  used  comparatively  little  in 
America.  The  processes  used  by  different  manufacturers  differ  in 


FIG.   636.— MODERN   STORAGE   BATTERIES. 


detail,  but  all  may  be  described  in  general  terms.    The  basis  of  each 
plate  is  a  "grid"  or  plate  of  solid  pure  lead,  with  sometimes  the  ad- 


ELECTROCHEMISTRY.  153 

dition  of  a  little  antimony  to  make  it  harder.  This  plate  is  cast,  rolled 
or  cut  into  a  shape  that  gives  a  large  amount  of  surface,  a  common 
method  being  to  make  a  series  of  parallel  grooves.  The  grooves  are 
then  filled  with  active  material,  either  by  the  mechanical  application 
of  lead  oxide  or  by  the  action  of  acids  which  attack  the  surface  of  the 
metal  until  the  interstices  are  filled  with  a  spongy  mass  of  lead  oxide. 
After  being  thoroughly  washed  to  remove  the  oxidizing  solution, 
the  plates  are  set  up  in  a  dilute  solution  of  sulphuric  acid  (Ha  SO), 
often  known  as  "oil  of  vitriol ;"  the  alternate  plates  are  connected  and 
current  is  sent  through  from  one  set  of  plates  to  the  other ;  current  is 
then  cut  off  and  the  plates  are  connected  through  a  resistance  so  as 
to  deliver  current  and  become  discharged;  current  is  then  sent 
through  them  again  for  a  longer  time  and  they  are  again  discharged. 
After  this  operation  has  been  repeated  several  times,  the  plates  are 
formed  and  are  ready  for  use.  Small  cells  are  sent  out  from  the 
factory  ready  for  filling  with  acid  and  for  charging.  For  larger 
cells,  such  as  used  in  central  stations,  the  plates  are  entirely  separate 
and  are  set  up  and  connected  where  the  battery  is  to  be  used.  In  a 
battery  for  central  station  use,  all  of  the  positive  plates  of  one  cell 
are  "burned"  or  fused  to  a  lead  connecting  strip,  to  whi-ch  also  are 
fused  all  of  the  negative  plates  of  the  next  cell ;  in  this  way  all  of  the 
cells  are  rigidly  connected  into  one  series.  In  smaller  batteries  it 
is  common  to  have  the  corresponding  plates  of  each  cell  fused  to  a 
common  terminal  strip,  and  the  different  cells  are  coupled  by  means 
of  lead  covered  bolts,  or,  in  some  cases,  by  rubber  covered  flexible 
copper  cables  bolted  to  the  lead  terminals. 

637.     Explain  the  action  of  the  storage  cell 

The  chemistry  of  the  lead  storage  cell  is  somewhat  complicated 
and  physicists  are  not  entirely  agreed  about  what  actually  takes 
place.  The  most  commonly  accepted  theory  is  as  follows :  When 
the  cell  is  thoroughly  charged,  the  positive  plate  is  covered  with  lead 
per  oxide  (PbO*),  while  the  negative  plate  is  covered  with  spongy 
metallic  lead.  (In  speaking  of  storage  batteries  the  positive  plate 
is  where  the  charging  current  enters  and  the  discharge  current 
leaves  the  cell).  During  the  discharge,  the  lead  at  the  negative  plate 
unites  with  the  sulphuric  acid  (H^SO*),  forming  lead  sulphate 
(PbSO),  and  setting  hydrogen  free.  At  the  positive  plate,  the  lead 
peroxide  (PbO)  unites  with  the  hydrogen  set  free  from  the  negative 
plate  and  forms  a  lower  oxide  (PbO)  and  water;  this  lower  oxide 
then  unites  with  sulphuric  acid  and  forms  lead  sulphate  (PbSO*) 
and  water  (Hz  O).  As  the  cell  becomes  discharged,  the  acid  thus 


154 


ELECTRICAL   CATECHISM. 


unites  with  the  plates  and  water  is  formed,  so  that  the  density  or 
specific  gravity  of  the  electrolyte  falls  from  about  1.22,  when  fully 
charged,  to  about  1.18  when  discharged  to  the  lowest  safe  limit. 
When  the  cell  is  charged  by  the  passage  of  current  in  the  opposite 
direction,  the  above  processes  are  reversed.  When  the  voltage  is 
allowed  to  fall  below  1.8  volts  per  cell,  the  lead  sulphate  is  liable  to 
change  into  an  insoluble  form,  which  reduces  the  capacity  of  the 


2.4 
2.3 
2.2 
2.1 

>2 

^^ 

^~- 

^-* 

^--—  ' 

^.     — 

—      — 



.—  -— 

^  ^  ' 

7. 

123456          789         10        11        12 
HOURS. 

FIG.  637A.— CHARGING  CURVE. 


a.a 

2.1 

1.8 

1  7 

s^ 

"*^^*-^ 

*~       ^. 

—  ^-^^— 

—         » 



'  -^ 

-**~-  ^ 

^^^. 

*\ 

x 

> 

\ 

\ 

9 


567 

HOURS. 
FIG.   637B.—  DISCHARGING   CURVE. 


10         11 


12 


cell,  and  the  cell  is  then  said  to  be  "sulphated."  Care  should  there- 
fore be  taken  not  to  allow  the  battery  to  become  discharged  too  far. 
When  the  cell  is  being  charged,  the  necessary  voltage  rises  from  2.2 
volts  to  about  2.5  volts  per  cell.  When  the  cell  is  fully  charged,  the 
further  application  of  current  breaks  up  the  water  of  the  solution, 
liberating  hydrogen  gas  at  the  negative  and  oxygen  gas  at  the 
positive.  These  gases  reduce  the  amount  of  the  solution  in  the  cell, 
both  by  the  decomposition  of  the  water  and  by  the  acid  which  is 
carried  off  in  spray  by  the  gases.  This  spray  is  also  carried  off  if  the 
charging  current  is  too  strong,  so  that  the  active  material  on  the 
electrodes  is  affected  at  the  surface  much  faster  than  nearer  the 
center  of  the  plate.  On  the  other  hand,  if  the  charging  current  is  less 
than  about  one-fifth  of  the  normal  current,  the  insoluble  sulphate  is 


ELECTROCHEMISTRY, 


155 


apt  to  form  and  so  injure  the  plates.  The  normal  rate  of  charging 
is  about  8  amps,  per  square  foot  of  total  surface  of  the  positive  plates. 
If  the  cell  is  charged  or  discharged  too  fast,  the  action  is  liable  to  be 
unequal  on  different  parts  of  the  plates,  and  so  to  cause  the  plates 
to  bend  or  "buckle,"  since  the  active  material  expands  during  dis- 
charge. 

638.     Hoiv  is  a  storage  battery  charged? 

Storage  batteries  are  almost  always  charged  from  dynamos  giving 
nearly  constant  E.M.F.  When  only  a  few  cells  are  to  be  charged, 
they  may  be  connected  across  a  continuous  current  incandescent 
lighting  circuit,  enough  lamps  being  in  series  with  them  to  give  the 
desired  current.  When  a  battery  of  fifty  or  more  cells  is  to  be 
charged,  connection  is  made  directly  with  the  dynamo.  In  beginning 
the  charge,  the  dynamo  should  be  adjusted  to  give  from  2  per  cent  to 


POSITIVE  LINE 


INCANDESCENT  LAMPS   IN    MULTIPLE 


FIG.    638.-CHARGING   BATTERY    FROM    INCANDESCENT    LIGHTING 

CIRCUIT. 

5  per  cent  higher  E.M.F.  than  that  of  the  batteries.  Both  terminals 
of  the  dynamo  should  then  be  connected  to  the  corresponding  ter- 
minals of  the  battery,  positive  to  positive  and  negative  to  negative. 
After  a  half  hour  or  so,  the  voltage  of  the  dynamo  may  be  raised  so 
as  to  increase  the  charging  current  to  the  normal  amount,  and  then 
when  the  battery  begins  to  gas,  its  voltage  will  have  risen  enough  to 
reduce  the  current.  In  central  station  practice,  where  the  same 
dynamos  must  supply  current  to  the  lines  at  constant  voltage  and  at 
the  same  time  charge  a  battery,  it  is  necessary  to  provide  means  for 
obtaining  two  different  voltages.  When  the  current  required  on  the 
lines  is  small  compared  with  that  required  by  the  batteries,  the  volt- 
age of  the  dynamo  may  be  adjusted  to  that  required  by  the  batteries, 
and  an  adjustable  resistance  may  be  inserted  in  the  main  line  to  re- 
duce the  line  voltage  to  that  required  by  the  circuit.  This  is  com- 
monly done  on  railway  trains,  where  the  dynamo  sometimes  charges 
the  batteries  and  at  the  same  time  furnishes  current  for  lighting  the 


156 


ELECTRICAL  CATECHISM. 


lamps.  On  the  other  hand,  when  the  battery  is  a  smaller  part  of  the 
entire  load,  provision  is  often  made  for  an  auxiliary  dynamo  to  raise 
the  voltage  of  the  current  used  for  charging  the  battery.  Such  a 


FIG.   638B.— BATTERY   SWITCHBOARD   FOR   ISOLATED   PLANT. 

dyi.  ono  is  called  a  "booster,"  and  may  be  driven  by  the  same  power 
that  operates  the  main  dynamos,  although  it  is  more  often  driven 
by  an  electric  motor.  The  battery  is  usually  connected  with  the 
booster  through  end  cell  switches. 


ELECTROCHEMISTRY.  157 

639.  How  can  the  positive  and  negative  terminals  of  a  battery 
or  dynamo  be  distinguished? 

In  the  storage  battery  the  positive  plates  have  a  brownish  or 
chocolate  appearance,  while  the  negative  plates  are  gray  or  slate 
colored.  The  polarity  of  a  battery  or  dynamo  may  be  tested  (as  ex- 
plained in  No.  311)  by  connecting  the  terminals  to  two  pieces  of  lead 
or  other  metal  dipped  in  water  which  has  a  slight  amount  of  acid; 
bubbles  of  gas  will  appear  at  both  pieces,  there  being  the  most  at  the 
terminal  connected  with  the  negative  terminal  of  the  battery  or  dy- 
namo. If  the  test  metals  are  of  lead,  the  piece  connected  with  the 
positive  terminal  will  become  coated  with  a  brown  deposit,  and  that 
connected  with  the  negative  terminal  will  become  gray  or  slate  col- 
ored. An  easier  method  of  determining  polarity  is  to  use  a  voltmeter 
of  the  permanent  magnet  type  and  having  its  terminals  marked;  if 
the  positive  terminal  of  the  battery  is  connected  with  the  positive 
terminal  of  the  voltmeter  and  the  negative  to  negative,  the  voltmeter 
needle  will  be  deflected  in  the  right  direction,  otherwise  it  will  move 
backward.  The  chemical  paper  or  potato  test  (see  Nos.  312  and 
313)  is  also  convenient. 

640.  How  can  one  tell  whether  a  storage  battery  is  working 
properly? 

Experience  counts -for  much  in  such  work.  The  appearance  of  the 
plates  is  a  good  indication  of  the  condition  of  the  battery.  When  the 
color  is  uniform  and  of  the  proper  gray  and  brown  shades,  the  bat- 
tery is  probably  all  right.  The  acid  should  be  tested  with  a  hy- 
drometer occasionally  to  see  that  the  solution  is  of  the  right  strength, 
not  less  than  1.15  when  the  battery  is  discharged,  and  not  more 
than  1.27  when  it  is  fully  charged,  and  it  should  be  adjusted  by  the 
addition  of  dilute  acid  or  of  clear  water,  as  the  case  requires.  The 
standard  density  of  electrolyte  for  any  given  battery  is  generally 
specified  in  the  printed  directions.  The  solution  should  always  cover 
the  plates  |  in.  or  more.  The  cells  should  not  be  allowed  to  dis- 
charge lower  than  1.8  volts  per  cell,  and  in  charging  the  pressure 
should  not  be  carried  higher  than  2.5  volts  or  2.6  volts  per  cell.  If 
one  cell  is  found  to  give  lower  voltage  than  others,  it  is  probably 
caused  by  some  foreign  substance  falling  into  the  cell  and  bridging 
across  from  positive  to  negative  plate  and  so  discharging  the  cell, 
or  changing  the  acid.  Any  metal  falling  into  the  liquid  will  be  dis- 
solved and  secondary  actions  will  be  apt  to  injure  the  cell.  Some- 
times the  active  material  or  sulphate  shed  from  the  plates  is  allowed 
to  collect  at  the  bottom  of  the  cell  to  such  a  depth  as  to  touch  the 


158 


ELECTRICAL   CATECHISM. 


plates  and  so  cause  a  local  circuit  which  will  discharge  the  cell.  Such 
a  cell  should  be  cleaned  and  then  fully  charged  by  itself.  If  the 
plates  become  covered  with  a  whitish  scale,  the  cell  has  "sulphated" 
from  being  discharged  to  too  low  voltage,  or  from  the  acid  being 
too  strong ;  this  reduces  the  capacity  of  the  battery  and  may  be  re- 
moved by  charging  the  cells  for  a  longer  time  than  usual,  making 
them  boil  for  some  time  until  the  plates  resume  their  normal  color. 
The  makers  will  generally  supply  full  instructions  for  the  care  of 
their  cells. 

641.     How  is  the  current  from  a  battery  regulated? 

Batteries  are  generally  designed  to  supply  a  circuit  at  constant 
E.M.F.,  and  the  current  is  then  controlled  by  the  resistance  of  the 
line  and  attachments.  As  the  battery  becomes  discharged,  its  pres- 
sure gradually  drops  off.  The  curve  in  Fig.  6^b  shows  how  the 
pressure  drops  when  the  discharge  current  is  kept  constant  at  the 


lttlfflttttlllll'1'ltt 


FIG.    641.— DIAGRAM    OF    SWITCHBOARD    FOR    STORAGE    BATTERY 
ON  THREE-WIRE  SYSTEM. 


ELECTROCHEMISTRY.  159 

normal  capacity  of  the  battery.  In  central  station  batteries  it  is  usual 
to  provide  enough  cells  to  supply  the  required  voltage  when 
all  are  discharged  down  to  the  safe  limit  of  1.8  volts  per  cell. 
The  cells  at  one  end  are  proyjded  with  terminals  leading  to  a 
switch,  so  that  the  total  number  of  cells  in  the  working  circuit 
may  be  varied  at  will.  At  the  beginning  of  the  discharge, 
when  all  the  cells  are  fully  charged  and  each  cell  furnishes 
about  2.2  volts,  it  takes  only  fifty  cells  to  furnish  no  volts;  but  as 
the  cells  discharge,  their  voltage  falls,  and  it  is  necessary  to  move 
the  "end  cell"  discharge  switch  so  as  to  cut  in  another  cell.  A  new 
end  cell  is  then  cut  in  for  each  drop  of  two  volts  in  the  total  pressure. 
Since  these  end  cells  are  in  use  a  shorter  time  than  the  main  battery, 
they  become  charged  sooner  and  it  is  necessary  to  have  also  an  end 
cell  charging  switch.  At  the  beginning  of  the  charge,  all  of  the 
end  cells  are  in  circuit ;  the  one  on  the  end  was  discharging  for  the 
shortest  time,  and  so  it  becomes  fully  charged  before  any  others,  and 
should  be  cut  out  of  the  circuit;  the  next  cell  likewise  becomes 
charged  soon  and  is  cut  out,  and  so  on  with  all  of  the  end  cells  until 
at  the  end  of  the  charge  only  the  cells  of  the  main  battery  are  in  cir- 
cuit. By  having  separate  end  cell  switches  for  charging  and  for  dis- 
charging, it  is  possible  to  keep  the  battery  connected  with  the  main 
circuit  both  while  it  is  charging  and  while  it  is  discharging.  In  the 
case  of  batteries  for  lighting  railway  trains  and  for  some  other  pur- 
poses, there  is  no  need  for  end  cells,  since  the  load  on  the  battery  is 
usually  greatest  at  the  early  evening  when  the  battery  is  fully 
charged  and  drops  off  as  the  battery  discharges.  In  telephone  ex- 
changes, the  battery  should  be  kept  well  charged,  so  that  the  dis- 
charge never  reaches  the  part  of -the  curve  where  the  pressure  drops 
rapidly. 

642.     For  what  purposes  are  storage  batteries  used? 

They  are  used  largely  for  replacing  the  more  expensive  primary 
cells  formerly  used  for  operating  telegraph,  telephone,  fire  alarm  and 
police  systems,  for  operating  small  motors,  medical  coils  and  similar 
portable  apparatus,  for  operating  motors  and  lights  on  automobiles, 
cars  and  boats;  for  steadying  fluctuating  loads  on  central  stations 
and  isolated  plants  for  furnishing  light  and  power;  for  increasing 
the  maximum  capacity  of  dynamo  plants  operated  by  steam  or  water 
power ;  to  furnish  a  reserve  power  for  emergency.  Storage  batteries 
will  return  from  40  per  cent  to  80  per  cent  of  the  energy  supplied 
in  charging,  and  thus  are  valuable  auxiliaries  in  electric  power 
plants  where  the  load  varies  through  wide  limits,  the  batteries  being 


160 


ELECTRICAL   CATECHISM. 


charged  when  the  load  is  small  and  when  charging  power  can  be 
developed  at  little  additional  cost,  and  supplying  current  when  the 
load  is  greatest  and  when  the  value  of  additional  power  is  highest. 
When  the  load  fluctuates  rapidly,  as  is  the  case  with  street  railways 


FIG.  642.— CURVES  SHOWING  REGULATING  EFFECT  OF  STORAGE 
BATTERIES. 


and  electric  elevators,  the  battery  acts  as  a  sort  of  fly-wheel,  taking 
a  charge  when  the  load  is  small  and  the  pressure  high,  and  delivering 
current  when  the  load  is  heavy  and  the  pressure  drops.  The  accom- 
panying curves  illustrate  such  action  of  a  battery. 

643.     In  what  ways  does  electricity  cause  chemical  action? 

As  noted  in  No.  600,  the  current  may  cause  chemical  action  di- 
rectly by  electrolysis,  or  indirectly  by  its  heating  effect.  An  electric 
current  has  no  chemical  effect  while  passing  through  a  solid  con- 
ductor ;  when  current  passes  through  any  liquid  conductor  other  than 
an  elementary  substance,  such  as  a  melted  metal,  the  liquid  is  de- 
composed and  chemical  action  appears  at  one  or  both  electrodes. 
Gases  do  not  seem  to  conduct  current  as  it  is  supposed  to  flow  in  a 
solid  conductor,  but  rather  carry  electricity  by  convection,  the  atoms 
or  ions  carrying  positive  and  negative  charges  between  the 


ELECTROCHEMISTRY.  161 

electrodes.  (See  No.  644).  In  some  cases,  especially  where  the 
gases  are  hot,  there  seems  to  be  an  electrolytic  action  similar  to  that 
occurring  in  liquids,  causing  the  combination  or  separation  of  atoms 
to  form  new  compounds. 

644.  What  is  an  ion? 

All  substances  are  composed  of  very  minute  particles  known  as 
molecules,  which  in  turn  are  composed  of  still  smaller  elementary 
particles  called  atoms.  Until  recently  the  molecule  has  been  looked 
upon  as  being  the  smallest  particle  of  matter  that  could  exist  alone 
or  in  a  free  state,  the  atoms  never  remaining  free,  but  always 
uniting  with  other  atoms  of  the  same  or  of  different  elements,  so 
as  to  form  molecules.  Recent  physical  investigations  indicate  that 
in  every  solution  there  exist  members  of  free  atoms  or  groups  of 
atoms  smaller  than  molecules,  which  are  called  ions.  The  ions  tend  to 
appear  at  the  electrodes,  those  at  the  anode  being  called  anions-,  those 
at  the  cathode  being  called  cations.  Each  ion  is  believed  to  carry 
a  static  charge,  by  which  it  is  attracted  to  the  oppositely  charged 
electrode.  A  theory  which  is  now  coming  into  general  acceptance  is 
that,  in  the  case  of  elementary  gases,  the  passage  of  an  electrical  dis- 
charge breaks  up  the  atoms,  which  have  heretofore  been  considered 
as  the  ultimate  particles  of  matter,  into  positive  and  negative  ions, 
the  negative  ion  being  only  a  small  chip  forming  about  one  one-thou- 
sandth part  of  the  original  atom,  while  the  remainder  of  the  atom 
becomes  the  positive  ion ;  each  of  these  charged  particles  may  gather 
around  itself  a  number  of  molecules  and  travel  toward  the  electrode. 
Thus,  in  an  elementary  gas,  such  as  hydrogen  or  oxygen,  an  ion  may 
be  a  small  chip  of  an  atom,  or  the  remainder  of  the  atom,  or  either 
of  these  together  with  a  group  of  other  attracted  molecules.  In 
general,  an  ion  may  be  said  to  be  a  small  charged  particle  of  matter. 
Many  recent  investigations  strengthen  the  theory  that  all  "  elements  " 
are  modifications  of  one  fundamental  unit  which  is  electricity. 

645.  What  is  electrolysis? 

Electrolysis  is  the  decomposition  of  conductors  by  the  passage 
of  current,  occurring  at  the  surface  in  contact  with  the  liquid 
conductor.  When  the  liquid  contains  a  metallic  compound  in  solu- 
tion, the  metal  is  usually  deposited  on  the  cathode  and  the  remain- 
ing part  of  the  compound  is  deposited  at  the  anode.  The  general 
rule  is  that  metals  travel  with  the  current.  In  many  cases  there  are 
secondary  reactions,  so  that  the  ions,  originally  liberated  by  the 
current,  form  other  combinations  before  they  can  be  secured  at  the 
electrodes.  In  many  cases  electrolysis  is  undesirable  and  would  be 


162  ELECTRICAL   CATECHISM. 

prevented  if  practicable,  as  in  the  case  of  damage  to  underground 
metal  pipes  by  railway  currents. 

646.  What  are  the  laivs  of  electrolysis? 

(i.)  The  amount  of  chemical  action  is  equal  at  all  parts  of  a  cir- 
cuit ;  that  is,  the  chemical  action  vis  independent  of  the  location  in  the 
circuit,  provided  the  current  is  equal  in  all  parts. 

(2.)  The  amount  of  an  ion  liberated  in  a  given  time  is  pro- 
portional to  the  strength  of  the  current. 

(3.)  The  amount  of  an  ion  liberated  at  an  electrode  in  one  sec- 
ond is  equal  to  the  strength  of  the  current  multiplied  by  the  "electro- 
chemical equivalent"  of  the  ion. 

The  first  two  laws  mean  that  the  chemical  effect  produced  is  pro- 
portional to  the  quantity  of  current  multiplied  by  the  time,  so  that 
10  amps,  acting  for  one  hour  produce  the  same  result  as  5  amps, 
acting  for  two  hours.  Explanation  of  the  third  law  involves  some 
chemical  knowledge  of  the  elements.  The  second  and  third  laws 
are  named  after  their  discoverer,  Faraday. 

647.  What  is  meant  by  the  "electro-chemical  equivalent?" 

The  electro-chemical  equivalent  of  an  element  is  the  amount  lib- 
erated by  one  coulomb.  Experiment  shows  that  one  coulomb  (i 
amp.  flowing  for  one  second)  will  liberate  0.000010384  grams  of  hy- 
drogen from  water  or  other  compound.  As  hydrogen  is  the  lightest 
element  known,  its  weight  is  taken  as  unity,  or  the  basis  of  com- 
paring the  weights  of  atoms  of  other  elements.  The  various  ele- 
ments are  also  classified  according  to  their  "valency,"  or  the  power 
they  have  of  uniting  with  other  elements.  Thus  one  atom  of  oxygen 
will  unite  with  two  atoms  of  hydrogen  to  form  one  molecule  of  water 
(HzO),  and  oxygen  is,  therefore,  said  to  be  a  dyad  or  to  have  a 
valency  of  two.  Other  elements  have  a  valency  of  one,  two,  three,  or 
even  four.  The  electro-chemical  equivalent  of  an  element  is  there- 
fore the  atomic  weight  multiplied  by  0.000010384  and  divided  by  the 
valency. 

648.  Explain  the  table  of  electro-chemical  equivalents. 

The  table  gives  a  list  of  the  more  common  elements  arranged  al- 
phabetically. The  second  column  gives  the  abbreviation  used  for 
each  element.  The  third  column  gives  their  valency,  that  is,  the  num- 
ber of  atoms  of  hydrogen  or  other  monad  element  with  which  one 
atom  of  the  element  will  unite.  The  fourth  column  gives  the  atomic 
weights  of  the  elements,  taking  the  weight  of  hydrogen  as  a  unit. 
The  fifth  column  gives  the  chemical  equivalent,  which  equals  the 


ELECTROCHEMISTRY. 


163 


atomic  weight  divided  by  the  valency.  The  sixth  column  gives  the 
electro-chemical  equivalent,  that  is,  the  number  of  grammes  of  each 
element  that  would  be  liberated  from  a  compound  by  I  amp.  of  cur- 
rent acting  for  one  second.  The  seventh  column  gives  the  number 
of  grammes  liberated  by  I  amp.  acting  for  one  hour,  this  being  equal 
to  3600  times  the  amount  liberated  in  one  second.  The  eighth  col- 
umn gives  the  number  of  pounds  of  each  element  liberated  by  I  amp. 
acting  for  one  hour.  The  ninth  and  tenth  columns  give  the  number 
of  ampere-hours  necessary  to  liberate  I  Ib.  or  i  kg.  In  all  of  the 
cases  it  should  be  remembered  that  the  ampere-hours  are  the  product 
of  current  by  time,  and  may  be  made  up  of  any  number  of  amperes 
multiplied  by  the  corresponding  number  of  hours; 

ELECTRO-CHEMICAL    EQUIVALENTS 


Element 

Sym- 
bol 

Val 
ency 

Atomic 
Weight. 

Chcm. 
Equiv. 

Electro 
Chemical 
Equivalent. 

|iS 

g  £  o 
g«S 

2    4>    «> 

S&! 

Wi 

Ampere 
Hours 
per  Pound 

se-§ 

M* 
&l 

Aluminum 

Al 

3 

27.5 

9.1 

.0000945 

0.339 

0.745 

1333. 

2950 

Sb 

3 

122. 

4D  66 

.000414 

1.490 

3285 

304 

671 

Arsenic    ..  

As 

1 

75. 

25. 

.000259 

0.932 

2055 

487. 

1073 

Barium  .... 

Ba 
Bi 

2 
3 

137. 
210. 

685 
70. 

.000709 
000725 

2.552 
2608 

5.627 
575 

178. 
174 

392. 
383 

B 

3 

81. 

1033 

000107 

0.385 

0  849 

1179 

2598 

Br 

1 

79.8 

79.75 

.0008^8 

2.1)82 

6  59 

152. 

335 

Cd 

2 

112. 

55.8 

.000578 

2.(79 

453 

218 

481 

Calcium     .  .... 

Ca 

2 

40. 

19.95 

.000207 

0.744 

1.64 

610. 

1343. 

Carbon 

c 

4 

12. 

3. 

.000031 

O.llJs 

0  -'44 

410 

8950 

Chlorine 

Cl 

1 

35.4 

35.37 

.000767 

1.322 

2915 

843. 

756 

Chromium...-  

Cr 

6 

52.5 

8.75 

.000091 

0.326 

0.7  2J 

1390. 

3068. 

Cobalt                

Co 

2 

58.8 

29.3 

.000305 

1.099 

242 

413 

910 

Copper  (cuprous), 
(cupric).. 

Cu 
Cu 

F 

1 
2 

1 

63 
63. 

19. 

63. 
31.5 

19. 

.000654 
.OOOJv*7 

.000197 

2.355 
1.178 

0.708 

5.192 
2.C96 

1.562 

193. 
S85. 

640. 

425. 
849. 

1413. 

Gold               —     .. 

Au 

3 

196.2 

65.4 

.000679 

2.445 

5391 

186. 

409 

Hydrogen  .... 

H 

1 

1. 

1. 

.00001038 

0.037 

0.082 

12.30. 

26750. 

I 

1 

1265 

126.53 

.001314 

4.730 

10^3 

96. 

211 

Iron  (ferrous)  

Iron  (ferric)  
Lead       

Fe 

Fe 
Pb 

2 

3 
2 

55.9 

55.9 

206.4 

27.95 

1863 
103.2 

.000290 

.000194 
.001072 

1.045 

0.697 

3.858 

.2.304 

1.536 
8.506 

434. 

651. 
118. 

257. 

1436. 
259. 

Li 

1 

7. 

7. 

.000072 

0.261 

0  575 

1739. 

3C30 

Magnesium  ._.„,_ 

Mg 

2 

23.9 

23.94 

.000124 

0.448 

0.987 

1014. 

2*35. 

Mn 

3 

55. 

18.27 

.000189 

0.581 

1.283 

779. 

1722. 

Mercury  (ous)  
(ic)  
Nickel 

If 

1 
2 
2 

199.8 
199.8 
586 

199.8 
99.9 
29.3 

.002075 
.OU1037 
.000304 

7.469 
3.735 
1.085 

16.47 
8.234 
2415 

61. 

122. 
414 

134. 

268. 
913 

N 

3 

14. 

4.67 

.000049 

0.175 

0385 

2598. 

5728. 

o 

2 

16. 

7.9S 

.000083 

0.298 

0.658 

1520. 

3852. 

P 

3 

31. 

10.8 

.000107 

0.384 

0.849 

1178. 

2604. 

Platinum  .  

Pt 

4 

197. 

49.25 

.000510 

1.832 

4.040 

248. 

546 

Potassium       .  ... 

K 

1 

39. 

39.04 

.000405 

1.459 

3.218 

111. 

685. 

Silicon 

Si 

2 

28. 

14. 

.000145 

0.522 

1.15 

870. 

1918 

Silver    

Ag 

1 

107.7 

107.66 

.001118 

4.025 

8.873 

113. 

249. 

Na 

1 

23. 

22.99 

.000239 

0.860 

1.895 

528 

1184 

s 

2 

32. 

16. 

.000166 

0.596 

1.315 

760 

1677 

Tin  (stannous)  
*'   (stannic)  
Zinc  

Sn 
Sn 
Zn 

2 
4 
2 

117.8 
117.8 
64.9 

589 
29.45 
32.45 

.000612 
.000306 
.000337 

2.202 
1.101 
1.213 

4.851 
2.427 
2.674 

206. 
412. 
374. 

454. 

908. 
824. 

164  ELECTRICAL   CATECHISM. 

atomic  weight 

Electro-chemical  equivalent  =  • ; —       —  X  0.000010^84. 

valency 

=  grammes  per  coulomb. 
=  grammes  per  ampere-second. 
Pounds  per  1000  amp.-hours  =  grm.  per  coulomb  X  3600  X 


=  el.  chem.  equiv.  X  793$.         453-4 

649.  W hat  are  the  laws  of  electrolytic  action  other  than  those  of 
Faraday? 

(4.)     Liberated  ions  appear  at  the  electrodes  only. 

(5.)  Every  electrolyte  is  decomposed  into  two  portions,  an  anion 
and  a  cation,  which  may  be  themselves  either  elements  or  compounds. 
Thus  water  (HaO)  is  decomposed  into  hydrogen  and  oxygen;  but 
when  current  is  sent  through  a  solution  of  copper  sulphate  (CuSO«), 
copper  is  liberated  at  the  cathode,  while  the  "sulphion"  (SO*) 
travels  to  the  anode,  with  which  it  usually  unites  to  form  a  sulphate. 

(6.)  In  binary  compounds  (those  composed  of  two  elements) 
and  most  metallic  solutions,  the  metal  is  deposited  at  the  cathode; 
thus  in  electroplating,  the  object  to  be  plated  is  made  the  cathode. 

(7.)  Aqueous  solutions  of  salts  of  the  alkaline  metals  (potas- 
sium, sodium,  lithium,  caesium,  rubidium  and  ammonium)  and  of 
the  alkaline  earths  (calcium,  barium  and  strontium)  deposit  no 
metal,  but  evolve  hydrogen  owing  to  the  secondary  action  of  the 
metal  upon  the  water.  Thus,  when  current  is  sent  through  a  solu- 
tion of  common  salt  (NaCl)  in  water,  chlorine  gas  is  given  off  at 
the  anode ;  but  instead  of  metallic  sodium  appearing  at  the  cathode, 
it  unites  with  the  water  as  fast  as  liberated  from  the  salt  and  forms 
sodium  hydrate  (NaOH)  or  caustic  soda,  and  in  turn  liberates 
hydrogen,  which  escapes  as  a  gas. 

(8.)  Metals  can  be  arranged  in  a  definite  series  according  to 
their  electrolytic  behavior,  the  more  oxidizable  metals  being  more 
strongly  electro-positive,  but  the  order  varies  with  the  nature, 
strength  and  temperature  of  the  solution  used  (see  No.  602).  Each 
metal,  when  electrolyzed  from  its  compound,  behaves  as  a  cation  in 
preference  to  one  more  electro-negative.  This  is  the  basis  of  methods 
of  electrolytic  refining  of  metals,  such  as  copper.  On  the  other  hand, 
alloys,  such  as  brass,  may  be  deposited  electrolytically  by  using  solu- 
tions such  that  the  component  metals  dissolve  with  equal  E.M.F's. 

(9.)  Several  metals  will  be  deposited  at  once  from  a  solution  of 
mixed  salts,  if  the  current  strength  is  so  great  that  the  solution  near 
the  cathode  becomes  weak.  For  example,  in  refining  copper,  the 
silver  and  other  impurities  remain  in  the  solution  while  the  copper 


ELECTROCHEMISTRY.  165 

alone  is  deposited,  unless  the  current  strength  increases  or  the 
amount  of  copper  in  the  solution  near  the  cathode  becomes  weak  in 
copper,  when  the  greater  resistance  causes  so  high  difference  of 
potential  that  the  silver  is  deposited  with  the  copper  on  the  cathode. 

( 10.)  For  each  electrolyte  a  minimum  E.M.F  is  requisite,  without 
which  complete  electrolysis  will  not  occur,  though  temporary  polari- 
zation may  exist.  In  such  a  case,  a  small  current  may  flow  for  a  short 
time,  but  the  polarization  immediately  causes  a  counter  E.M.F., 
which  stops  the  further  passage  of  current.  (See,  however  (n), 
below.) 

(n.)  When  both  electrodes  are  of  the  same  metal  in  their  own 
salt,  there  is  no  opposing  E.M.F.,  since  no  net  chemical  work  is  done, 
and  the  smallest  E.M.F.  will  effect  deposition.  For  example,  in  the 
Edison  chemical  meter,  electrodes  of  amalgamated  zinc  are  immersed 
in  a  solution  of  zinc  chloride  and  connected  around  a  shunt  of  low 
resistance.  The  current  divides  between  the  shunt  and  the  "bottle," 
and  a  definite  proportion  of  the  total  current  passing  through  the 
electrolytic  cell  transfers  a  definite  amount  of  zinc  from  the  anode 
to  the  cathode,  no  net  chemical  work  being  done  as  the  zinc  is  sim- 
ply transferred  from  one  plate  to  the  other.  This  also  is  important 
in  electrolytic  refining  of  metals. 

(12.)  Where  the  ions  are  gases,  pressure  affects  the  conditions 
but  slightly.  Thus,  I  amp.  passing  through  acidulated  water  liber- 
ates 0.0000103  grams  of  hydrogen  each  second,  no  difference  whether 
the  hydrogen  escapes  into  the  open  air  or  into  a  vacuum  or  into  a 
receiver  under  the  pressure  of  many  pounds  per  square  inch.  The 
E.M.F.  necessary  to  effect  decomposition  increases  to  some  extent 
with  the  pressure  on  the  gas,  but  the  given  current  liberates  the  same 
weight  of  gas  in  any  case. 

(13.)  The  chemical  work  done  in  a  cell  is  proportional  to  the 
minimum  E.M.F.  necessary  for  polarization,  and  any  additional 
E.M.F.  simply  represents  the  energy  used  in  heating  the  cell. 

(14.)  Ohm's  law  holds  good  for  electrolytic  conduction.  Thus, 
the  resistance  of  a  cell  varies  directly  with  the  distance  between  the 
electrodes  and  inversely  with  their  size.  The  total  E.M.F.  necessary 
for  deposition  consists  of  two  parts,  one  equal  to  the  E.M.F.  of 
polarization  and  the  other  equal  to  the  product  of  current  by  ohmic 
resistance  of  the  cell. 

(15.)  Secondary  reactions  may  result  in  (a)  the  ions  decom- 
posing, as  SO*  may  decompose  into  SOa  and  O ;  (b)  the  ions  reacting 
on  the  electrodes,  as  in  the  case  of  storage  batteries ;  (c)  the  ions  be- 
ing liberated  in  an  abnormal  state,  as  "when  oxygen  is  set  free  in  the 
state  of  ozone. 


166  ELECTRICAL  CATECHISM. 

(16.)  An  electric  current  sometimes  seems  to  act  as  a  directing- 
force  to  set  in  operation  chemical  forces  previously  latent.  Thus,  the 
damage  to  underground  pipes  seems  in  some  cases  to  be  due  only 
secondarily  to  the  electric  currents  from  the  street  railways  which 
put  into  action  chemical  forces  latent  in  the  soil. 

650.  How  much  work  is  required  to  send  current  through  an 
electrolyte? 

The  work  consists  principally  of  three  parts,  although  there  may 
be  other  sources  of  small  losses  when  the  electrolyte  is  not  of  uni- 
form density.  Part  of  the  work  appears  as  heat,  corresponding  to 
the  resistance  loss  in  a  metallic  conductor,  being  equal  to  the  product 
of  current  by  the  true  resistance  of  the  electrolyte.  A  second  loss 
occurs  at  the  surfaces  of  the  electrodes  where  the  molecules  are  re- 
arranging themselves,  a  process  called  "sweeping"  which  repre- 
sents an  actual  loss ;  the  work  lost  in  sweeping  increases  very  rapidly 
for  a  few  minutes  when  the  current  first  begins  to  pass,  and  then  is 
proportional  to  the  strength  of  the  current.  The  third  part  of  the 
work  is  that  done  in  effecting  the  chemical  changes  in  the  electrolyte, 
and  is  strictly  proportional  to  the  current;  the  chemical  work 
causes  an  E.  M.  F.  which  is  independent  of  the  strength  of  the 
current  and  depends  upon  the  chemical  changes  that  occur;  when 
this  E.  M.  F.  is  in  a  direction  to  increase  the  current,  the  electrolytic 
cell  becomes  a  battery;  when  it  tends  to  reduce  the  current,  it  is 
called  a  counter-electromotive  force. 

651.  What  is  meant  by  polarization  in  an  electrolyte? 

The  polarization  is  the  cause  of  the  C.  E.  M.  F.  just  mentioned.  It 
may  be  transient,  so  that  it  disappears  when  the  current  ceases,  or 
it  may  be  permanent  and  be  capable  of  causing  a  current  to  flow  after 
the  former  main  E.  M.  F.  is  removed.  The  most  striking  example 
is  the  storage  battery  which  is  "charged"  by  the  current  from  some 
other  source  and.  later  becomes  itself  a  secondary  source  of  current. 

652.  Does  polarization  always  occur  when  current  passes  'through 
an  electrolyte? 

Polarization  always  occurs  when  there  is  any  net  chemical  work 
done.  When  both  electrodes  are  of  the  same  metal  and  the  elec- 
trolyte is  a  "salt"  or  compound  of  that  same  metal,  the  current  sim- 
ply transfers  metal  from  one  plate  to  the  other ;  there  is  no  net  chemi- 
cal work  done  and  no  polarization  occurs.  This  fact  is  the  basis 
of  the  Edison  chemical  meter  and  some  other  voltameters. 


ELECTROCHEMISTRY.  167 

653.  Is  polarisation  a  desirable  phenomenon? 

In  many  instances  it  is  undesirable,  in  others  it  is  the  foundation 
upon  which  the  application  of  electricity  is  made.  In  the  battery  or 
chemical  cell  for  generating  electricity  one  polarization  causes  the 
E.  M.  F.,  while  another  may  introduce  an  undesirable  resistance. 
Polarization  is  closely  identified  with  most  of  the  electro-chemical 
processes  of  manufacture, 

654.  Where  can  further  treatment  of  electrochemistry  be  found? 

"  Practical  Electrochemistry,"  by  B.  Blount ;  "  Notes  on  Electro- 
chemistry," by  F.  G.  Wiechmann ;  "  Experimental  Electrochemis- 
try," by  N.  M.  Hopkins ;  "  Radioactivity,"  by  E.  Rutherford ;  "  The 
New  Knowledge,"  by  R.  K.  Duncan ;  "  Primary  Batteries,"  by  H.  S. 
Carhart;  "  Primary  Batteries,"  by  W.  R.  Cooper;  "  Storage  Battery 
Engineering,"  by  L.  Lyndon ;  "  Electric  Smelting  and  Refining,"  by 
W.  Borchers  and  W.  G.  McMillan.  See  also  sections  on  "  Electro- 
chemistry "  in  "  Standard  Handbook  for  Electrical  Engineers,"  and 
in  Foster's  "  Electrical  Engineer's  Pocket  Book." 


CHAPTER  VI. 


MAGNETISM. 

700.  What  is  the  relation  between  electricity  and  magnetism? 
Magnetism  is  sometimes  considered  as  one  form  of  electricity,  that 

is,  electricity  in  rotation.     (See  Nos.  4  and  8.)    It  is  believed  that 
all  magnetism  is  due  originally  to  the  action  of  electric  currents. 

701.  What  is  the  difference  between  magnetism  and  electro- 
magnetism? 

Electro-magnetism  generally  refers  to  magnetic  fields  that  are 
maintained  by  the  more  or  less  continuous  action  of  electric  currents. 


FIG.  701.— A  SIMPLE  ELECTROMAGNET. 

Magnetism  is  a  more  general  term  and  sometimes  refers  to  magnetic 
fields  existing  without  the  immediate  influence  of  current. 

702.  What  is  meant  by  a  magnetic  line  of  force? 

The  term  is  used  either  to  indicate  the  direction  of  the  force  or 
as  a  measure  of  the  strength  of  the  magnetic  field.  The  lines  of 
force  are  considered  as  coming  out  from  the  north  pole  of  a  magnet 
(the  end  that  would  point  toward  the  north  if  free  to  turn),  and  as 
returning  into  the  south  pole.  (For  the  value  of  the  line  of  force 
as  a  unit  of  magnetic  measurement,  see  Nos.  241  to  246.) 

703.  What  connection  is  there  between  a  current  and  a  magnet? 
A  magnetic  field  exists  around  a  current,  much  like  that  about  a 

magnet. 

704.  What  is  meant  by  a  magnetic  field? 

This  can  be  illustrated  by  a  simple  experiment.  Scatter  iron 
filings  or  chips  upon  a  piece  of  glass  or  cardboard  placed  over  a  mag- 
net, and  then  strike  the  cardboard  gently.  The  filings  will  gradually 


MAGNETISM. 


169 


arrange  themselves  in  well-defined  lines,  somewhat  as  shown  in 
the  figure.    The  arrangement  of  the  iron  particles  shows  that  there 


FIG.  704.— FIELD  OF  FORCE  AROUND  BAR  MAGNET. 

is  some  sort  of  force  in  the  space  around  the  magnet,  a  force  that 
acts  in  well- defined  directions  and  that  is  strongest  near  the  poles 
or  ends  of  the  magnet.  If  a  compass  needle  or  a  magnetized  steel 
needle  suspended  by  a  hair  or  thread  is  moved  about  near  the  mag- 
net, it  will  point  in  the  direction  of  these  lines,  showing  that  there  is 
a  field  of  force  all  around. 

705.  How  can  it  be  shown  that  there  is  a  magnetic  field  due  to 
the  current? 

A  simple  way  to  show  that  the  current  is  surrounded  by  a  mag- 
netic field  is  to  bring  a  compass  needle  near  a  wire  that  is  carrying 
a  current.  If  the  wire  is  horizontal,  and  the  needle  is  placed  over  or 
under  the  wire,  it  will  be  ratated  by  the  current  and  will  stand  about 
at  right  angles  to  the  wire  (the  wire  should  run  north  and  south,  so 
that  the  needle  would  naturally  be  parallel  to  the  wire).  If  the  wire 
is  vertical,  the  needle  will  turn  so  as  to  be  tangent  to  the  wire.  If 


CURRENT  UP 
FIG.  705.— MAGNETIC  NEEDLE  DEFLECTED  BY  CURRENT. 

the  needle  is  moved  around  the  wire,  it  will  take  different  positions 
with  its  axis  always  tangent  to  a  circle.  A  current  of  10  amp.  is 
strong  enough  to  show  these  effects  easily. 


170 


ELECTRICAL  CATECHISM, 


706.  How  can  the  magnetic  field  about  a  current  be  shown  by 
iron  filings? 

Pass  a  wire  vertically  through  a  card  or  smooth  board  upon  which 
iron  filings  or  small  chips  have  been  sprinkled.  Send  a  current  of 
about  20  amp.  through  the  wire,  and  at  the  same  time  slightly  jai 
the  card  so  that  the  iron  bits  can  move  easily.  They  will  gradually 


FIG.  706.-MAGNETIC  FIELD  AROUND  CURRENT. 

arrange  themselves  in  more  or  less  perfect  circles,  as  shown  in  the 
figure.  If  one  can  not  easily  get  a  current  of  20  amp.,  he  can  get 
almost  as  good  results  by  passing  the  wire  through  the  hole  several 
times,  making  a  coil  of  large  diameter  so  that  the  returning  wires  will 
not  affect  the  field  seriously.  With  practice  and  care  in  selecting 
iron  bits  of  the  right  size,  one  can  obtain  clearly  defined  circles  about 
the  wire. 

707.  Why  is  there  a  clear  space  close  around  the  wire  where  there 
are  no  iron  bits? 

Near  the  wire  the  field  is  so  strong  that  the  iron  bits  are  drawn 
to  the  wire  and  so  leave  the  space  clear  for  some  little  distance  awav. 

708.  How  is  the  magnetic  field  about  a  wire  affected  by  forming 
the  wire  into  a  coil? 

Some  of  the  lines  will  still  surround  the  individual  wires,  but  most 
of  them  will  unite  and  pass  through  the  entire  coil,  as  seen  in  the 
figure,  taken  from  a  photograph.  The  field  is  seen  to  be  strong 
through  the  center  of  the  coil,  so  that  the  iron  bits  arrange  them- 
selves in  quite  definite  lines,  while  outside  the  coil,  the  field  is  more 
scattered  and  is  not  strong  enough  to  collect  the  iron  bits  into  so 
definite  lines.  It  is  seen  that  the  magnet  field  is  strengthened  by 
the  current  in  each  wire ;  in  fact,  if  the  wires  are  close  together,  the 


MAGNETISM.  171 

total  strength  of  field  is  proportional  to  the  product  of  the  current 
by  the  number  of  wires.      A  coil  is  sometimes  called  a  solenoid 


FIG.   708.— MAGNETIC  FIELD   OF  SOLENOID. 

or  helix.     When  the  coil  is  bent  so  its  axis  or  core  forms  a  closed 
circuit,  the  coil  is  called  a  torus. 

709.     What  sort  of  field  exists  betzveen  the  poles  of  a  magnet f 
The  lines  of  force  go  from  one  pole  to  the  other  in  nearly  straight 
lines.     Between  parallel  surfaces  the  lines  are  straight.     The  lines 
of  force  beyond  the  parallel  surfaces  are  curves,  somewhat  as  shown 


FIG.  709.-FIELD   BETWEEN   POLES  OF  A   MAGNET. 

in  the  figure.  The  magnetic  force  becomes  less  and  less  intense  as  the 
distance  from  the  magnet  increases,  as  shown  by  the  lines  of  iron 
particles  being  less  and  less  distinct. 

710.     What  reason  is  there  for  believing  that  the  lines  of  force 
tend  to  follow  the  easiest  path  possible? 

This  is  a  conclusion  from  the  facts  that  particles  of  iron  are  drawn 


172  ELECTRICAL   CATECHISM. 

into  the  magnetic  field ;  that  the  magnetism  is  much  stronger  when 
its  path  is  largely  or  entirely  through  iron ;  and  that  the  lines  tend 
to  become  as  short  as  possible. 

711.  How  can  it  be  shown  that  the  lines  of  force  tend  to  become 
as  short  as  possible? 

This  is  illustrated  by  the  well  known  phenomena  of  attraction,  as 
when  a  magnet  attracts  its  keeper.  Familiar  examples  are  the  tele- 
graph sounder  and  the  electric  bell. 

712.  How  is  the  attraction  of  unlike  poles  and  the  repulsion  of 
like  poles  explained? 

There  is  no  complete  explanation  agreed  upon,  although  all  the 
actions  can  be  referred  to  simple  laws.  There  is  a  tension  along 
the  lines  of  force  and  a  pressure  at  right  angles  to  them ;  in  other 
words,  the  lines  tend  to  shorten  and  to  spread  apart  with  a  definite 
and  known  force. 

713.  With  how  much  force  do  the  magnetic  lines  tend  to  spread 
apart? 

The  spreading  force  measured  in  dynes  equals  gausses  divided  by 
8*  or  multiplied  by  0.04.  (See  Nos.  208,  242,  246.) 

714.  With  how  much  force  do    the    lines    tend    to    contract  or 
shorten  ? 

The  contractile,  measured  in  dynes,  equals  gausses  multiplied  by 
0.04,  similar  to  the  spreading  force. 

715.  How  can  one  calculate  the  force  with  which  a  magnet  at- 
tracts another  magnet  or  a  piece  of  iron? 

The  force  between  the  two  equals  the  area  of  the  common  surface 
multiplied  by  the  square  of  the  number  of  lines  of  force  per  square 
centimeter  divided  by  25.14,  or  multiplied  by  0.04. 


For  example,  suppose  two  pieces  of  iron  I  cm.  square  and  of  any  con- 
venient length  are  bent  into  half  circles,  and  that  the  current  is  sent 
through  a  coil  of  wire  wound  about  one  or  both  pieces  so  as  to  mag- 
netize the  iron  to  a  saturation  of  5000  lines  per  square  centimeter; 
the  force  at  each  joint  between  the  two  parts  of  the  ring  will  be 
0.04  X  i  X  5000  X  5000  —  0.04  X  25,000,000  =  1,000,000  dynes; 
the  total  force  will  be  the  sum  of  that  at  the  two  joints,  or  double  that 
at  one;  hence,  the  total  pull  is  2,000,000  dynes.  Since  981  dynes 


MAGNETISM.  173 

equal  the  weight  of  I  grm.,  the  total  pull  between  the  two  pieces  is 

2,000,000  -h  981  —  2024  grm., 
or  about  4.4  Ibs. 

716.  How  can  this  rule  be  modified  for  English  measures? 
Four  hundred  fifty-three  and  six-tenths  grammes  equal  one  Ib. 

Hence,  if  the  dimensions  and  saturation  are  given  in  centimeters,  the 
pull  (in  pounds)  at  each  pole  equals  the  area  multiplied  by  the  square 
of  the  number  of  lines  per  square  centimeter  divided  by  25.14  times 
981  times  453.6;  or  divided  by  11,183,000.  If  English  measures  are 
used  throughout,  the  rule  becomes:  The  pull  (in  pounds)  equals  the 
total  area  of  contact  (in  square  inches)  multiplied  by  the  square 
of  the  number  of  lines  of  force  per  square  inch  divided  by  11,183,000 
times  6.45  or  divided  by  72,134,000. 

717.  Give  an  example  of  calculating  the  pull  of  a  magnet. 
Suppose  the  magnet  is  made  of  iron  ij  ins.  x  ij  ins.,  bent  into 

U-shape  and  magnetized  to  a  saturation  of  40,000  lines  per  square 
inch.  The  area  of  the  iron  is  1.875  sq.  ins.  at  each  end.  The  pull 
at  each  end  is 

40,000  X  40,000  X   1-875 

=  41.6  Ibs. 

72,134,000 

The  total  pull  for  both  ends  will  then  be  twice  this,  or  83.2  Ibs. 

718.  Is  the  above  rule  for  pull  of  a  magnet  rigorously  correct? 
It  supposes  that  the  magnetism  is  uniformly  strong  over  the  whole 

end  of  the  magnet.  If  the  contact  between  the  magnet  end  and  the 
piece  it  is  lifting  is  equally  good  over  the  whole  surface,  the  mag- 
netism will  usually  be  equally  distributed  and  the  rule  in  its  simple 
form  holds  true.  If  the  magnetism  is  stronger  in  some  places,  then 
the  rule  becomes  more  complicated. 

719.  How  can  a  magnetic  traction  table  be  calculated? 

It  is  convenient  to  have  corresponding  values  for  lines  per  square 
centimeter,  lines  per  square  inch,  grammes  pull  per  square  centi- 
meter, and  pounds  pull  per  square  inch.  Since  a  square  inch  equals 
645  sq.  cm.,  the  second  column  is  derived  by  multiplying  figures  in 
the  first  column  by  6.45.  Since  (see  No.  715)  the  pull  in  grammes 
equals  the  square  of  the  number  of  lines  per  square  centimeter  mul- 
tiplied by  the  area  in  centimeters  and  divided  by  8  TT  times  981,  the 
grammes  pull  per  square  centimeter  is  obtained  by  squaring  the  lines 
per  square  centimeter  and  dividing  by  24,655,  or  multiplying  by 
0.00004056.  Since  453.6  grms.  equal  I  Ib.  and  6.45  sq.  cm.  equal 
i  sq.  in.,  the  pounds  pull  per  square  inch  equals  the  square  of  the 


174 


ELECTRICAL   CATECHISM. 


number  of  lines  per  square  inch,  divided  by  8  times  981  times  453.6 
times  6.45,  or  lines  per  square  inch  squared  divided  by  72,134,000  or 
multiplied  by  0,00000001387.  Such  a  table,  abridged  from  Thomp- 
son, is  given. 

MAGNETIC  TRACTION  TABLE 


Lines  per  Sq.  Cm. 

Lines  per  Sq.  Inch 

Grammes  per  Sq.  Cm. 

Pounds  per  Sq.  Inch 

J,OCO 

6,450 

40.56 

•577 

2,000 

12,900 

.162.3 

2.308 

3,000 

19,350 

365.1 

5,19° 

4,OOO 

25,800 

648.9 

9.228 

5,ooo 

32,250 

1,014 

14-39 

6,000 

38,700 

1,460 

20.75 

7,000 

45,150 

1,987 

28.26 

8,000 

51,600 

2,596 

3695 

9,000 

58,050 

3,286 

46-72 

10,000 

64,500 

4,056 

57-68 

11,000 

70,950 

4,907 

69.77 

12,000 

77,400 

5,841 

83.07 

13,000 

83,850 

6,855 

97-47 

14,000 

90,300 

7,950 

ii3<i 

15,000 

96,750 

9,124 

129.7 

16,000 

103,200 

10,390 

147-7 

17,000 

109,650 

11,720 

166.6 

18,000 

Il6,IOO 

I3,HO 

186.8 

19,000 

122,550 

14,630 

208.1 

20,000 

129,000 

16,230 

230.8 

Lines  per  square  inch  =  6.45  X  lines  per  square  centimeter. 

B2 

Grammes  per  square  centimeter  = — — =•  =  0.00004056  X  lines 

OTT    X  9°* 
per  square  centimeter  squared. 

B2  (sq.  in.) 

Pounds  per  square  inch  = =  0.00000001387  X  lines  per 

72,134,000 

square  inch  squared, 

720.     How  can  a  traction  curve  be  made  and  used? 

A  convenient  one  is  made  by  taking  lines  of  force  per  square  cen- 
timeter for  ordinates  or  vertical  distances  and  grammes  per  square 
inch  of  surface  for  the  abscissae  or  horizontal  distances.  Using  the 
rule  in  No.  715,  calculate  the  pull  in  grammes  for  a  number  of 
values  of  the  lines  of  force  and  draw  a  curve  through  these  points. 
Then  the  pull  for  any  other  strength  of  field  may  be  taken  from  the 
curve  without  any  calculation.  For  example,  for  10,000  lines  of 
force  per  square  centimeter  the  pull  is  found  to  be  4056  grms. 


MAGNETISM. 


175 


for  each  square  centimeter  of  surface  of  the  pole;  for  14,000  lines 
of  force  the  pull  is  8000  grms.,  and  so  for  other  points.  A  similar 
curve  for  pounds  pull  per  square  inch  for  various  values  for  the  mag- 
netization may  be  calculated  from  the  rule  in  No.  716,  which  may 


Grm.per  sq.cm.  1,000  2,000  3,000  4,000  5,000  6,000  7,000  8,000  9,000  10,000  11,000  12,000  13,000  14,000  15,000 
Lbs.  per  sq.in.   20    40    60    80    100   120   140   160   180   200   220   240   260   280   300 
Lines  per  sq.in.  10,000      30,000      50,000      70,000      90,000      110,000     130,000     150,000 

FIG.  720.— MAGNETIC  TRACTION  CURVES. 

be  modified  to'suit.  The  same  curves  may  be  used  to  show  the  num- 
ber of  lines  per  square  inch  which  correspond  with  any  desired  num- 
ber per  square  centimeter :  Since  I  sq.  in.  equals  6.45  sq.  cm.,  if  a 
line  is  drawn  through  the  zero  and  through  a  point  which  represents 
10,000  lines  per  square  centimeter,  and  64,500  lines  per  square  inch, 
corresponding  values  may  be  found  for  any  other  value;  for  ex- 
ample, 14,000  lines  per  square  centimeter  equal  90,000  lines  per 
square  inch. 

721.  How  can  the  pull  in  pounds  be  obtained  from  the  curve 
^vhen  the  number  of  lines  per  square  inch  is  known? 

Suppose  there  are  90,000  lines  per  square  inch.  Follow  up  the 
ordinate  at  90,000  lines  per  square  inch  until  it  meets  the  diagonal 
straight  line,  which  it  does  at  14,000  lines  per  square  centimeter. 
Then  follow  along  the  horizontal  line  at  the  point  where  the  diagonal 
line  was  met,  until  the  curve  of  "pounds  per  square  inch"  is  met,  as 
at  the  point  indicated  by  the  heavy  line  upon  the  curve  sheet,  and 
then  follow  down  to  the  scale  for  "pounds  per  square  inch,"  when 


176 


ELECTRICAL   CATECHISM. 


it  is  found  that  a  magnetization  of  90,000  lines  per  square  inch  gives 
a  pull  of  112  Ibs.  per  square  inch  of  surface  of  the  end  of  the  magnet. 

722.  How  strongly  can  iron  be  magnetized? 

There  is  no  definite  limit.  Cast  iron  is  well  saturated  at  40,000 
lines  per  square  inch,  or  6200  per  square  centimeter.  Wrought  iron 
is  well  saturated  at  100,000  lines  per  square  inch,  or  15,500  lines  per 
square  centimeter. 

723.  What  is  meant  by  iron  being  saturated f 

When  iron  is  subjected  to  a  magnetizing  force,  such  as  a  current 
through  a  coil  of  wire  surrounding  the  iron,  the  magnetization  of 
the  iron  increases  very  rapidly  at  first  as  the  current  increases  from 
zero.  After  the  current  has  reached  a  certain  value,  any  increase 
has  less  and  less  effect.  This  is  illustrated  in  the  figure,  in  which 


120,000 
110,000 
100,000 
90,000 
80,000 
70,000 
60,000 
50,000 
40,000 
30,000 
20,000 
10,000 

S2> 

^ 

^^ 

7~~^ 

~~  —  • 

_—  — 

_  —  — 

—  

-  

- 

••          ~^ 

==«— 

/ 

/ 

{ 

_-^—  -J 

.  

.  



—1  —  — 

,-  •*• 

^^ 

^^ 

• 

.—  •  — 

x' 

?? 

I 

/ 

/ 

X 

100    200    300   400    500   600    700 


900    1000   1100   1200   1300  1400 
Am.Elec. 


FIG.  723.— MAGNETIC  SATURATION  CURVES  FOR  WROUGHT  AND 
CAST  IRON. 

horizontal  distances  represent  magnetizing  force,  and  vertical  dis- 
tances represent  the  resulting  magnetization  of  the  iron.  The  turn- 
ing point  in  each  curve  is  called  the  point  of  saturation,  being  at  about 


MAGNETISM.  177 

100,000  for  wrought  iron  and  40,000  for  cast  iron.      These  values 
vary  with  the  composition  and  hardness  of  the  iron. 

724.  What  is  meant  by  permeability? 

Permeability  is  the  conductivity  for  magnetic  lines  of  force.  In 
other  words,  it  is  a  measure  of  the  ease  with  which  magnetism  passes 
through  any  substance.  The  permeability  of  good  soft  wrought 
iron  is  sometimes  3000  times  that  of  air,  varying  with  the  quality  of 
the  iron.  It  also  becomes  less  as  the  intensity  of  the  magnetism  in- 
creases. The  permeability  of  cast  iron  and  cast  steel  is  from  fifty 
to  800  times  that  of  air.  Some  cast  steel  is  nearly  as  good  as 
wrought  iron.  (See  permeability  curves  in  Fig.  735.) 

725.  What  is  meant  by  "B"  and  "H"  as  applied  to  magnetism? 

"B"  refers  to  the  intensity  of  magnetization  through  some  mag- 
netic substance,  such  as  iron  or  nickle.  "H"  refers  to  the  mag- 
netizing force,  or  what  is  equal,  the  magnetization  that  would  exist 
if  there  were  no  iron  present.  For  example,  suppose  that  a  wooden 
ring  were  wound  with  a  number  of  turns  of  wire  and  that  a  certain 
current  were  sent  through  the  coil  so  that  fifty  lines  of  force  per 
square  centimeter  of  cross  section  would  be  sent  through  the  ring. 
Now  suppose  that  the  ring  had  been  iron  instead  of  wood  and  that 
the  permeability  of  the  iron  were  320,  then  there  would  be  320  times 
50,  or  16,000  lines  of  force  per  square  centimeter.  In  this  case  "H" 
would  equal  50,  and  "B"  would  equal  16,000.  "H"  is  also  used  some- 
times to  represent  loss  by  magnetic  hysteresis  (see  Nos.  760  to  765), 
and  is  commonly  used  to  denote  the  horizontal  component  of  the 
earth's  magnetism.  (See  No.  777.) 

726.  Is  there  any  definite  relation  between  a  current  and  the 
magnetisation  it  produces? 

There  is  a  perfectly  definite  relation  similar  to  that  expressed  by 
Ohm's  law  for  electric  circuits.  The  number  of  magnetic  lines  of 
force  in  any  case  equals  the  magnetizing  force  divided  by  the  mag- 
netic reluctance.  Magnetizing  force  corresponds  closely  with 
E.M.F.,  reluctance  corresponds  closely  with  resistance,  and 
magnetization  corresponds  with  current.  In  this  comparison  it  must 
be  remembered  that  no  energy  is  required  to  maintain  a  magnetic 
field  unless  it  is  doing  work,  otherwise  a  permanent  magnet  would 
be  a  source  of  perpetual  motion,  which  is  contrary  to  reason. 

727.  What  is  the  relation  of  current  to  magnetomotive  force? 
The     unit     of     M.M.F.     is     the     gilbert,     and     equals     that 


178  ELECTRICAL  CATECHISM. 

j 

produced  by  —  amp.  turn  (see  Nos.  241  to  250) ;  in  other  words, 

47T 

I  amp.  turn  sets  up  a  M.M.F.  of  12.57  gilberts. 

728.  How  is  an  electromagnet  designed  f 

There  are  a  number  of  methods,  all  based  on  the  law  that  the  mag- 
netization equals  the  magnetizing  force  divided  by  the  reluctance  of 
the  magnetic  circuit.  A  common  method  of  procedure  is  first  to 
decide  upon  the  number  of  magnetic  lines  of  force  that  are  to  be  sent 
through  the  circuit ;  then  decide  upon  the  intensity  of  magnetization 
desired  in  the  iron,  thus  fixing  the  area  of  the  iron;  the  length  of 
the  iron  circuit  is  then  estimated,  allowing  what  is  thought  to  be 
enough  room  for  the  coils;  then  calculate  the  number  of  ampere 
turns  necessary  to  produce  the  desired  magnetization;  then  decide 
the  current  to  be  used,  which  determines  the  number  of  turns  of  wire 
necessary. in  the  coils;  the  size  of  wire  is  then  determined;  this  de- 
termines the  amount  of  space  needed  for  the  coils,  and  so  corrects  the 
length  assumed  for  the  magnetic  circuit,  and  allows  a  still  closer  ap- 
proximation to  be  made  upon  the  number  of  turns  of  wire  and  the 
exact  size  of  the  coil. 

729.  What  determines  the  number  of  lines  of  force  necessary  in 
a  given  case? 

The  purpose  for  which  the  magnet  is  to  be  used.  For  a  lifting 
magnet,  the  rules  given  in  Nos.  715  to  723.  For  a  dynamo  or  motor, 
the  winding  and  speed  of  the  armature  fix  the  number  of  lines  of 
force,  while  the  dimensions  of  the  armature  fix  the  dimensions  of 
the  field  magnet  to  some  extent.  Allowance  for  leakage  must  be 
made,  since  not  all  the  lines  of  force  are  likely  to  go  through  the  path 
desired. 

730.  What  fixes  the  intensity  of  magnetisation  of  the  iron? 

In  some  cases  the  number  of  lines  of  force  and  also  the  area  of 
the  iron  are  fixed  by  other  conditions,  when  the  intensity  equals  the 
total  number  of  lines  divided  by  the  area.  For  example,  if  100,000 
lines  of  force  are  required  and  the  cross  section  of  the  magnet  is 
2  sq.  ins.,  the  intensity  of  magnetization  will  be  50,000  lines  per 
square  inch.  When  the  area  is  not  fixed  by  other  consideration, 
one  would  commonly  decide  to  use  an  intensity  of  magnetization  a 
little  below  the  "knee"  or  bend  of  the  magnetization  curve,  say  about 
40,000  lines  per  square  inch  for  cast  iron,  or  about  80,000  to  90,000 
for  wrought  iron  or  cast  steel,  since  such  intensity  gives  the  best 
economy  of  iron  and  copper. 


MAGNETISM.  179 

731.     Give  an  example  of  calculating  a  magnet. 

Suppose  it  is  desired  to  calculate  a  magnet  to  lift  500  Ibs.  Since 
the  pull  increases  as  the  square  of  the  magnetization,  it  is  desirable  to 
use  a  high  saturation,  say  100,000  lines  per  square  inch.  By  re- 
ferring to  the  table  or  curve  already  prepared,  it  is  seen  that  this 


FIG.  731.— LIFTING  MAGNET. 

would  lift  127  Ibs.  per  square  inch.  Dividing  500  by  127  gives 
3.94  as  the  area  of  the  pulling  surface.  Since  each  of  the  two  poles 
lifts  half,  the  area  of  each  polar  surface  should  be  half  of  3.94,  or  a 
little  less  than  2  sq.  ins.  To  give  a  reasonable  factor  of  safety,  this 
should  be  doubled  or  quadrupled. 

732.  What  fixes  the  length  of  the  iron  circuit  of  a  magnet? 

The  general  shape  desired  will  fix  the  length  approximately.  Suf- 
ficient space  must  be  allowed  for  the  magnetizing  coils,  which  or- 
dinarily take  a  space  from  one  to  four  times  the  length  of  the  di- 
ameter of  the  iron  core. 

733.  What  determines  the  number  of  ampere  turns  necessary  to 
magnetize  a  circuit  to  a  desired  intensity? 

This  is  determined  by  the  length  and  area  of  the  various  parts  of 
the  circuit  and  by  the  permeability  of  each  part,  by  the  general  equa- 
tion: 

Magnetomotive  force 

Magnetization  = =—. —      • 

Reluctance 

This  may  be  developed  in  detail  as : 

4-rrNI 
,         10        1.257  N  I  Au 

i~         nr 

Au 

in  which  ^>  is  the  number  of  lines  of  force,  N  is  the  number  of  turns 
of  wire,  /  is  the  current,  A  is  the  area  of  the  section  of  the  mag- 


180 


ELECTRICAL  CATECHISM. 


netic  circuit,  /  is  its  length  and  u  its  permeability,  all  in  C.G.S. 
units.  This  may  be  put  into  more  convenient  form  for  calculating 
the  number  of  ampere  turns  necessary  for  each  part  of  the  circuit  as 
follows : 

NI=        **      =.7955B-. 
i. 257  Au  u 

When  all  measurements  are  in  inches,  this  becomes 

<frl"  B"  1 ' 

N  I  ==^0.3132  =  0.31321— 

If  the  magnetic  circuit  is  uniform,  the  ampere  turns  may  be  cal- 
culated directly  from  this  formula.  If  there  are  a  number  of  unlike 
parts,  the  M.M.F.  for  each  part  may  be  calculated  by 
the  rule,  and  the  sum  of  these  gives  the  total  number  of  ampere  turns. 


200         400 


600 


Permeability 
800          1000         1200        1400         1600        1800        2000 


10,000 


20  40  60  80  100          120          140          160          180          200 

Ampere  Turns  per  Inch  =JC-^  1.257 

FIG.  735.— CURVES  OF  PERMEABILITY  AND  AMPERE  TURNS. 


MAGNETISM. 


181 


734.  Give  an  example  of  calculating  the  number  of  ampere  turns 
necessary  for  a  magnet. 

Take  the  case  of  the  lifting  magnet  discussed  in  No.  731.  Exam- 
ining the  curve  or  table  for  good  wrought  iron,  the  permeability  is 
found  to  be  about  450  at  the  assumed  saturation  of  100,000  lines  per 
square  inch.  Assume  a  total  length  .of  15  ins.  for  the  magnetic 
circuit.  Placing  the  values  in  the  last  formula  gives 
100,000  X  15 

N   I   =  =   0.3132 — =    1034 

for  the  ampere  turns  necessary  to  magnetize  the  iron.  In  the  case 
of  a  field  magnet  for  a  dynamo  or  motor,  the  calculation  would  be 
similar  except  that  it  would  be  more  complicated,  as  there  are  several 
parts  to  be  calculated. 

735.  Is  there  a  shorter  method  of  calculating  ampere  turns  by 
means  of  a  curve  or  table? 

Tables  and  curves  may  be  made  without  much  difficulty  if  the  per- 
meability curve  is  known  for  the  iron  to  be  used.  The  formulae  in 
No.  733  show  that  the  ampere  turns  necessary  for  i  cm.  equal  0.7955 
times  the  number  of  lines  per  square  centimeter  divided  by  the  per- 
meability, and  that  the  ampere  turns  necessary  for  I  in.  equal  0.3132 
times  the  number  of  lines  per  square  inch  divided  by  the  permeabil- 
ity. A  table  may  be  constructed  by  taking  the  permeabilities  cor- 
responding to  various  magnetizations  and  working  out  the  results 

AMPERE  TURNS  PER  INCH  LENGTH  OF  MAGNETIC  CIRCUIT. 
(From  Poole,  Am.  El., 


Lines  per 
Sq   Cm. 

Lines  per 
Sq.  Inch 

Cast  Iron 

Cast 
Steel 

Wrought 
Iton 

Sh^et  lion 

Air 

1,000 

6,45° 

2 

2,021 

2,000 

12,900 

4 

,  .  .  , 

4,042 

3,000 

19.350 

7 

i-5 

6,063 

4.OOO 

25,800 

12 

4 

2.5 

8,084 

5,000 

32,25o 

22 

5 

32 

10,  no 

6,000 

38,700 

50 

8 

6 

4.2 

12,130 

7,000 

45,^50 

85 

9 

7 

5-2 

14,150 

8,000 

51,600 

162 

In 

8 

6.4 

16,170 

9,000 

58,050 

228 

12 

10 

8.1 

18,190 

10,000 

64,500 

306 

14 

12 

10.2 

20,210 

11,000 

70,95° 

438 

17 

15 

13-5 

22.230 

12,000 

77,400 

2O 

!9 

17-5 

24,250 

13,000 

83,850 

26 

25 

24. 

26,270 

14,000 

90,300 

.... 

38 

36 

32. 

28.290 

15,000 

96,750 

.... 

60 

58 

50. 

30,320 

182  ELECTRICAL  CATECHISM. 

as  indicated.  The  accompanying  tables  and  curves  are  worked  out 
for  excellent  grades  of  wrought  and  cast  iron,  and  mild  cast  steel. 
For  close  accuracy,  curves  should  be  worked  out  for  the  particular 
kind  of  material  used.  Since  the  p.ermeability  of  air  is  unity,  the 
number  of  ampere  turns  necessary  to  send  the  magnetic  lines  through 
air  equals  0.7955  times  the  number  of  lines  per  square  centimeter 
for  each  centimeter  length  of  air  space,  or  0.3132  times  the  number 
of  lines  per  square  inch  for  each  inch  of  air  space. 

736.  What  iixes  the  current  to  be  used  in  a  magnet? 
Sometimes  the  current  is  already  determined,  as  in  the  constant 

current  used  for  series  arc  lighting.  On  telegraph  lines  the  current 
is  fixed  within  certain  limits  by  the  length  of  the  line,  the  number  of 
offices  and  the  number  of  cells  in  the  batteries.  When  the  magnetiz- 
ing coil  is  to  be  connected  directly  across  the  terminals  of  a  battery 
or  the  mains  of  a  constant  potential  system,  such  as  used  for  incan- 
descent lighting,  the  coil  must  have  sufficient  resistance  to  keep  the 
current  down  to  a  safe  strength.  The  current  through  the  field 
magnet  coil  of  a  series  motor  on  a  constant  potential  circuit,  such  as 
a  fan  motor  or  a  street  railway  motor,  is  governed  prin- 
cipally by  the  work  being  done  by  the  armature.  When  the  magnet 
is  that  of  a  shunt  dynamo  or  motor  (see  Nos.  1146  to  1149)5  tne  co^ 
should  be  so  calculated  that  only  a  small  proportion  of  the  total  cur- 
rent taken  by  the  machine  shall  pass  through  the  field  magnet  coil. 

737.  What  proportion  of  the  total  current  is  usually  taken  by  the 
fields  of  shunt  dynamos  and  motors  f 

It  varies  from  30  per  cent  or  more  in  small  sizes  (J  hp)  to  I  per 
cent  in  very  large  machines. 

738.  What  fixes  the  size  of  wire  to  be  used  in  a  magnet  coil? 
The  size  wire  is  determined  both  by  the  allowable  resistance  and 

by  the  current  to  be  carried.  As  noted  in  No.  417,  magnet  coils 
generally  have  the  wire  proportioned  for  about  850  to  1400  circ.  mils 
per  ampere.  Another  rule  is  to  allow  not  more  than  one-third  of  a 
watt  to  be  dissipated  per  square  inch  of  surface  of  the  coil. 

739.  How  is  the  zvinding  of  a  magnet  made  to  have  &  certain 
resistance? 

The  resistance  to  be  given  the  coil  is  determined  by  the  voltage  to 
be  applied  to  its  terminals  and  by  the  current  to  flow.  By  Ohm's  law 
the  resistance  equals  the  voltage  divided  by  current.  From  the 
size  of  the  iron  magnet  core,  the  average  length  of  a  turn  of  wire 
can  be  estimated  closely;  this  multiplied  by  the  number  of  turns, 


MAGNETISM.  183 

gives  the  total  length  of  the  wire.  The  size  of  wire  to  give  the  de- 
sired resistance  with  the  calculated  length  can  then  be  found  from 
a  wire  table  (see  No.  348),  or  it  may  be  calculated  from  the  rule 
given  in  No.  352. 

740.  How  can  the  permeability  of  iron  be  measured? 

There  are  a  number  of  different  methods.  The  most  accurate 
methods  involve  the  use  of  ballistic  galvanometers,  and  are  reliable 
only  with  skilled  observers.  Comparatively  simple  methods  have 
been  devised  by  Bidwell,  Thompson  and  others,  based  upon  the  at- 
traction between  pieces  of  iron  subjected  to  known  magnetomotive 
forces.  Bidwell  forms  the  iron  into  halves  of  a  ring  which  has 
definite  dimensions,  the  two  parts  fitting  closely ;  a  coil  of  wire  is 
wound  around  one-half  of  the  ring  and  the  attraction  between  the  two 
is  measured  for  various  values  of  current,  the  number  of  lines  of 
force  being  calculated  from  the  formula  given  in  Nos.  715  to  717, 
and  the  magnetizing  force  being  calculated  from  the  formula  given 
in  Nos.  725  and  733.  Thompson  has  arranged  a  similar  device  for 
testing  straight  rods,  called  the  permeameter. 

741.  What  is  the  permeameter? 

It  consists  of  a  large  block  of  soft  wrought  iron  in  which  is  a  coil 
of  wire.  The  test  piece  passes  through  a  brass  bushing  in  one  end 
of  the  large  block  and  through  the  coil,  its  end  being  carefully  sur- 
faced to  fit  the  block.  The  number  of  lines  of  force  through  the  test 
piece  are  calculated  from  the  pull  required  to  remove  the  test  piece. 


FIG.  741.-A  PERMEAMETER. 

The  reluctance  of  the  magnetic  circuit  is  practically  all  in  the  test 
piece,  and  the  entire  length  of  the  magnetic  circuit  may  be  considered 
to  be  that  of  the  test  piece  measured  between  the  ends  of  the  gap 
in  the  block,  an  element  of  uncertainty  being  the  reluctance  between 
the  test  piece  and  the  upper  part  of  the  block.  It  is  not  well  suited 
for  testing  sheet  iron. 


184 


ELECTRICAL  CATECHISM. 


742.  How  many  kinds  of  magnets  are  there? 

Magnets  may  be  classified  according  to  whether  the  magnetism 
is  permanent  or  is  temporary,  or  according  to  the  shape  of  the  cir- 
cuit. 

743.  How  are  magnets  classified  according  to  shape? 

The  more  common  forms  are  bars,  horse  shoes,  multipolar  and 


FIG.   743A.— IRONCLAD 
MAGNET. 


FIG.  743s.— MAGNET  CIRCUIT  OF  MULTIPOLAR 
DYNAMO. 

ironclad.  Distinctions  are  also  made  between  those  with  salient 
poles  and  those  with  consequent  poles.  It  is  not  always  easy  to  de- 
cide in  which  class  a  given  magnet  will  fall.  A  bar  magnet  is  straight 
and  the  lines  of  force  come  out  from  one  end  and  pass  to  the  other 
through  the  air,  as  indicated  in  the  figures  with  Nos.  704  and  708.  A 
horseshoe  magnet  is  shaped  something  like  the  letter  "U,"  an  ex- 
ample being  shown  in  figure  with  Nos.  731  and  771.  An  example 
of  an  ironclad  magnet  is  shown  in  the  accompanying  figure,  743a. 
Ironclad  field  magnets  are  often  used  for  electric  motors  for  use 
on  street  cars  and  in  other  exposed  places.  (Figs.  743C  and  743d.) 
Multipolar  magnets  are  frequently  used  on  large  dynamos  and 
motors,  multipolar  meaning  ''many  poled."  (See  No.  1121.) 

744.     What  is  the  difference  between  salient  and  consequent  poles? 

All  of  the  lines  of  force  through  salient  poles  come  through  the 
same  magnetizing  coil,  while  those  through  consequent  poles  come 
partly  through  one  coil  and  partly  through  another.  It  is  not  al- 
ways easy  to  distinguish. 


MAGNETISM. 


185 


745.     What  is  meant  by  magnetic  leakage? 

It  is  found  that  some  of  the  magnetic  lines  of  force  do  not  remain 
in  the  iron,  but  jump  across  through  the  air  or  other  surrounding  ma- 
terial for  part  of  the  way.  This  is  because  air  and  all  other  sub- 
stances have  a  certain  amount  of  magnetic  conductivity,  and  because 
iron  has  at  least  some  reluctance.  The  magnetic  circuit  may  be 
compared  with  an  electric  circuit  immersed  in  a  poorly  conducting 
medium,  such  as  water. 


FIGS.  743c  AND  743D.— IRON-CLAD  MOTORS. 

746.  Is  there  any  way  to  insulate  against  magnetic  leakage? 
Magnetic  leakage  may  be  reduced  by  making  the  desired  path  of 

low  reluctance,  so  that  there  is  little  tendency  for  the  lines  to  seek 
other  paths.  No  substance  has  yet  been  found  that  will  serve  as 
a  magnetic  insulator.  (See  also  Nos.  747  to  757.) 

747.  What  is  meant  by  the  "stray  magnetic  field"  about  a  dynamo 
or  motor? 

Hold  a  screwdriver  or  wrench  near  different  parts  of  the  iron 
frame  of  a  dynamo  or  motor  that  is  running,  and  it  will  be  attracted 
more  or  less  strongly  at  some  positions.  Wherever  the  iron  is  at- 
tracted, it  indicates  that  some  of  the  magnetism  leaves  the  field  mag- 
net at  that  point.  Some  styles  of  machine  have  much  stronger  stray 
fields  than  others. 


186  ELECTRICAL  CATECHISM. 

748.  What  harm  is  there  in  a  stray  field  about  a  machine f 

It  is  liable  to  magnetize  watches  so  that  they  will  not  keep  cor- 
rect time;  it  is  liable  to  attract  tools  or  pieces  of  iron  into  the  ma- 
chine, with  danger  of  getting  against  moving  parts  or  making  elec- 
trical connections  where  not  wanted;  the  stray  field  may  cause 
heating  in  the  pulley.  Cases  have  been  known  where  dynamos  mag- 
netized the  engine  so  that  the  governor  would  not  act  properly. 
Stray  magnetism  means  that  part  of  magnetic  field  is  not  useful 
in  generating  E.M.F.  in  the  armature,  and  so  means  waste  of  energy 
in  the  field  coils. 

749.  How  can  magnetism  be  removed  from  a  watch  f 

One  way  is  to  place  the  watch  in  the  center  of  a  coil  of  wire 
through  which  an  alternating  current  is  passing.  Then  draw  the 
watch  away  slowly  while  the  coil  is  still  carrying  current.  The  mag- 


FIG.   749.— WATCH   DEMAGNETIZING   COIL. 

netic  parts  of  the  watch  thus  have  the  magnetism  rapidly  reversed, 
each  magnetization  being  weaker  than  the  reverse  one  preceding.  The 
coil  may  be  placed  in  series  with  an  incandescent  lamp  and  be  con- 
nected to  the  electric  lighting  circuit.  If  the  latter  gives  continuous 
instead  of  alternating  current,  it  may  be  made  alternating  by  means  of 
a  reversing  switch  or  commutator  connected  around  the  coil. 

Another  way  is  to  hold  the  watch  on  a  twisted  string  and  bring  it 
near  a  pole  piece  of  a  dynamo ;  then  move  it  away  while  it  is  twist- 
ing rapidly. 

750.  Is  it  possible  to  prevent  watches  from  becoming  magnet- 
ised? 

Common  watches  must  be  kept  away  from  strong  magnetic  fields 
or  they  are  liable  to  become  magnetized.  Some  of  the  so-called . 
' 'non-magnetic  watches"  have  the  springs  and  balance  wheel  made 
of  some  non-magnetizable  substance,  such  as  palladium,  phoshor- 
bronze  or  other  alloy.  Some  of  these  are  not  affected  even  by  com- 
paratively strong  fields. 


MAGNETISM,  18? 

751.  Do  the  "magnetic  shields"  sometimes  sold  for  watches  keep 
out  the  magnetism ? 

They  may  if  made  thick  enough,  but  as  generally  made  they  are 
useless  except  to  the  seller.  No  substance  has  yet  been  discovered 
which  will  not  allow  magnetic  lines  to  pass  through  it.  Conse- 
quently, the  only  way  to  prevent  magnetism  from  getting  in  any 
space  is  to  keep  it  away  from  magnetic  fields.  One  way  to  do  this 
is  to  provide  an  easier  path  for  whatever  stray  magnetism  may  be 
about.  The  magnetic  shields  are  intended  to  be  made  of  some 
metal,  such  as  soft  iron  or  nickel,  which  is  so  good  a  conductor  that 
any  magnetic  lines  of  force  will  pass  around  through  the  iron  rather 
than  through  the  space  inclosed.  A  thick  iron  pocket  or  case  is  there- 
fore the  best  anti-magnetic  shield,  but  it  must  be  thick  to  be  of  any 
value. 

752.  Why    do    many    switchboard   instruments    have    cast-iron 
cases? 

Partly  to  furnish  so  good  a  path  for  any  stray  magnetic  field  from 
the  dynamos  or  bus-bars  that  it  will  pass  through  the  case  and  not 


FIG.  752.— IRON-CLAD  VOLTMETER. 

enter  the  space  inside  where  it  would  interfere  with  the  correct 
working  of  the  instrument. 

753.  What  is  meant  by  ammeters  or  voltmeters  being  "iron- 
clad?" 

Some  instruments  have  iron  cases  to  make  the  instrument  free 
from  disturbances  by  outside  magnetic  fields. 

754.  Are  ordinary  ammeters  and  voltmeters  affected  by  the  stray 
field  about  a  dynamo  or  motor? 

Some  excellent  instruments  may  be  affected  so  that  the  measure- 
ments are  several  per  cent  too  high  or  too  low  if  the  instrument  is 
within  10  ft.  of  an  electric  machine. 


188  ELECTRICAL  CATECHISM. 

755.  How  can  one  tell  whether  or  not  an  instrument  is  affected 
by  a  stray  field? 

By  turning  the  instrument  half-way  around,  while  it  carries  cur- 
rent, so  that  the  stray  magnetism  will  be  relatively  in  the  opposite 
direction.  If  it  is  affected  by  the  field,  the  instrument  will  read 
higher  in  one  position  than  in  the  other.  The  average  of  two  read- 
ings, taken  in  opposite  positions,  is  the  correct  reading. 

756.  Can  magnetic  leakage  from  a  dynamo  or  motor  be  pre- 
vented? 

Not  entirely.  After  a  machine  is  finished,  it  is  practically  impos- 
sible to  reduce  the  magnetic  leakage,  although  it  may  be  made  greater 
by  placing  iron  near  the  poles.  The  stray  field  may  sometimes  be 
confined  to  the  space  near  the  dynamo  by  surrounding  the  machine 
by  a  thick  iron  case. 

757-     Can  magnetic  leakage  be  reduced  by  proper  design? 

By  making  the  regular  desired  path  through  the  armature  of  very 
low  magnetic  reluctance,  a  larger  proportion  of  the  lines  from  the 
field  will  pass  through  the  armature  and  fewer  will  stray.  The 
lines  follow  the  easiest  path.  The  armature  core  should  contain 
plenty  of  soft  iron,  and  the  iron  core  should  come  as  close  to  the 
pole  pieces  as  is  possible  while  allowing  sufficient  mechanical  clear- 
ance. Some  machines,  especially  those  with  toothed  armatures,  re- 
quire a  wider  air  space  than  others,  in  order  to  prevent  excessive 
sparking  at  the  brushes. 

758.  How  can  magnetic  leakage  be  detected  and  measured? 

If  a  compass  needle  is  brought  near  a  magnet,  the  needle  will  be- 
gin to  vibrate  more  or  less  rapidly  according  to  the  strength  of  the 
magnetic  field.  The  rapidity  of  the  vibration  and  the  amount  the 
needle  is  turned  from  pointing  north  and  south  is  an  indication  of 
the  strength  of  the  field.  Sometimes  the  strength  of  the  leakage 
field  is  indicated  by  the  strength  with  which  the  magnet  attracts  a 
piece  of  iron  or  steel  such  as  a  screwdriver  or  wrench.  The  num- 
ber of  magnetic  lines  of  force  coming  out  from  definite  parts  may 
sometimes  be  measured  by  the  induction  through  a  coil  of  wire 
wound  around  the  point  to  be  tested,  the  induction  being  measured 
by  a  ballistic  galvanometer  or  by  a  suitable  voltmeter  when  the  mag- 
netizing current  is  cut  off  or  reversed. 

759.  What  is  the  difference  between  a  permanent  magnet  and  an 
electromagnet? 

An  electromagnet  is  one  that  depends  for  its  power  upon  the  con- 
tinuous action  of  an  electric  current.  A  permanent  magnet'  is  one 


MAGNETISM. 


189 


that  retains  the  magnetization  after  the  compelling  cause  has  been 
removed.  The  softness  of  the  iron  and  the  shape  of  the  magnetic 
circuit  have  much  to  do  with  the  proportion  of  the  original  magnet- 
ization that  is  retained.  Soft  wrought  iron  retains  very  little  mag- 
netism, especially  when  the  magnetic  circuit  is  partly  through  air 
or  other  non-magnetic  material.  The  harder  the  iron  or  steel,  the 
larger  proportional  part  of  the  maximum  magnetization  will  be  re- 
tained. 

760.     What  is  meant  by  hysteresis ? 

Hysteresis  is  a  sort  of  friction  between  the  molecules  of  iron,  or 
of  any  other  magnetic  substance,  which  causes  the  magnetization  to 
lag  behind  the  magnetizing  current.  The  result  is  that  when  the 
magnetizing  current  increases  the  magnetization  of  the  iron  also  in- 
creases, but  when  the  current  decreases  the  magnetization  does  not 
drop  off  exactly  with  the  current ;  but  for  each  value  of  the  decreas- 
ing current  the  corresponding  magnetization  is  greater  than  it  was 
for  the  same  value  of  the  increasing  current.  This  is  illustrated  in 
the  figure.  When  the  current  (or  ampere-turns  in  the  coil)  in- 
creases from  zero  to  a  value  represented  by  the  length  of  the  base 
line  from  O  to  A,  the  magnetization  of  the  iron  core  increases  by 
an  amount  represented  by  the  ordinate  or  vertical  distance  from  O 
to  D.  In  the  same  way,  a  current  OB  causes  a  magnetization  OG, 
and  a  current  OC  causes  a  magnetization  OH.  Now,  as 
the  current  becomes  less,  the  magnetization  corresponding 
to  a  given  current  is  the  same  as  before,  until  the  current 
becomes  less  than  OB,  and  the  resulting  magnetism  is  less  than 


O       A  O  C 

FIG.  760.— MAGNETIZATION   CURVES   SHOWING  HYSTERESIS. 


enough  to  "saturate"  the  iron.  As  the  current  falls  below  this 
amount,  the  magnetism  no  longer  falls  off  so  rapidly  as  it  increased, 
so  that  when  the  current  has  decreased  to  OA,  the  magnetism  has 


190  ELECTRICAL  CATECHISM. 

only  dropped  to  OF,  instead  of  to  OD.  When  the  current  drops  to 
zero,  some  of  the  magnetism  still  remains  for  a  time,  and  may  have 
the  value  of  OE  or  less.  The  effect  of  hysteresis  in  instruments  is 
that  such  instruments  as  depend  upon  the  attraction  or  repulsion  of 
a  soft  iron  core  by  a  coil  do  not  give  the  same  reading  for  an  in- 
creasing current  as  for  a  decreasing  current,  except  when  the  core 
is  magnetically  saturated. 

761.  What  is  meant  by  remanencef 

This  is  a  term  sometimes  used  to  denote  the  amount  of  magnetism 
that  remains  after  the  current  is  removed.  The  amount  of  .mag- 
netism represented  by  the  line  OE  in  Fig.  760  is  the  remanence  for 
the  particular  case  there  shown.  This  is  more  often  called  residual 
magnetism. 

762.  What  is  meant  by  coercive  force f 

Coercive  force  is  the  M.M.F.  necessary  to  remove  the  residual 
magnetism.  This  is  sometimes  called  "  coercitive  force."  ;  ..* 

763.  What  is  meant  by  hysteresis  loss?    . 

When  iron  is  subjected  to  a  magnetizing  force  that  varies  through 
a  regular  cycle,  increasing  from  zero  to  a  maximum  and  then  de- 
creasing to  zero,  reversing  and  increasing  to  a  maximum  and  de- 
creasing again  to  zero,  the  magnetization  follows  more  or  less  close- 


1  I  I  I  I  I  1  I  I  I  I  1  I  I  I  I  I  !  I  I  I 

30     26      22      18     14      10  8  6  4 


FIG.  763.-HYSTERESIS  CURVES  FOR  IRON  WIRE, 

Iy  the  changes  in  the  current.      In  such  a  case  the  magnetization 
curve  shown  in  Fig.  760  becomes  doubled,  so  as  to  make  a  syta- 


MAGNETISM.  191 

metrical  curve.  The  accompanying  figure  shows  curves  for  a  soft 
iron  wire,  the  dotted  curve  showing  the  hysteresis  curve  when  the 
iron  was  hard,  and  the  solid  curve  showing  the  magnetization  cycle 
after  the  same  wire  was  carefully  annealed.  It  requires  a  cer- 
tain amount  of  energy  to  magnetize  and  demagnetize  iron.  If  the 
magnetization  followed  the  same  curve  in  ascending  and  descending, 
no  energy  would  be  required,  since  the  magnet  would  restore  as  much 
to  the  circuit  in  decreasing  as  it  absorbed  when  increasing.  The 
difference  between  the  two  curves  represents  the  amount  of  loss. 
This  hysteresis  loss  becomes  important  in  armatures  of  dynamos 
and  motors  and  in  the  iron  cores  oi  transformers,  since  the  mag- 
netism passes  through  many  cycles  each  second,  and  a  large  amount 
of  energy  may  be  absorbed  in  hysteresis. 

764.  What  becomes  of  the  energy  lost  by  hysteresis? 

It  is  converted  into  heat.  An  armature  or  transformer  whose 
iron  core  is  hard  will  heat  much  faster  than  one  whose  iron  is  care- 
fully annealed,  since  the  area  of  the  hysteresis  cycle  is  larger.  The 
chemical  purity  of  the  iron  has  much  also  to  do  with  the  amount  of 
the  hysteresis  loss. 

765.  How  is  the  hysteresis  loss  calculated? 

The  watts  lost  per  cubic  centimeter  of  iron  equal  the  number  of 
complete  cycles  of  magnetization  per  second  multiplied  by  the  num- 
ber of  lines  of  force  per  square  centimeter  of  section  raised  to  the 
1. 6th  power  multiplied  by  a  constant  and  divided  by  10,000,000.  The 
constant  factor  varies  from  0.002  for  the  best  wrought  iron  to  0.016 
for  good  cast  iron,  and  higher  figures  for  hard  iron,  going  as  high 
as  0.075  for  very  hard  steel.  For  an  example,  take  the  case  of  the 
armature  of  a  continuous  current  dynamo.  The  armature  is  gen- 
erally magnetized  to  about  15,000  lines  per  square  centimeter;  this 
figure  raised  to  the  i.6th  power  equals  4,806,000.  If  it  is  a  bipolar 
machine  making  1800  r.  p.  m.,  or  30  r.  p.  s.,  and  if  the  constant  is 
taken  as  0.002,  the  hysteresis  loss  per  cubic  centimeter  of  the  arma- 
ture core  is 

0.002  X  30  X  4,806,000 
• — •  =  0.029  watts. 

10,000,000 

With  alternators  the  magnetization  is  likely  to  be  about  7000,  and 
with  transformers  it  is  likely  to  be  about  9000,  lines  per  square  cen- 
timeter. The  loss  by  hysteresis  is  entirely  distinct  from  the  loss 
that  comes  from  eddy  or  foucault  currents. 


192  ELECTRICAL   CATECHISM. 

766.  Is  residual  magnetism  desirable  for  any  purpose? 

The  residual  magnetism  is  the  source  of  power  of  all  permanent 
magnets.  Even  in  electromagnets,  residual  magnetism  is  frequently 
desirable,  as  for  example  in  the  field  magnets  of  dynamos.  The 
residual  magnetism  gives  an  initial  field,  so  that  a  small  E.M.F.  is 
caused  when  the  machine  comes  up  to  speed,  and  this  small  E.M.F. 
causes  a  small  current  to  circulate  through  the  magnet  coils  so  that 
the  machine  soon  "picks  up"  if  everything  is  all  right. 

767.  What  determines  the  amount  of  residual  magnetism  that  is 
retained  in  a  magnetic  circuit? 

It  depends  upon  the  original  strength  of  magnetization,  upon  the 
hardness  and  composition  of  the  iron  or  steel,  upon  the  shape  of  the 
magnetic  circuit  and  upon  how  nearly  it  is  completely  made  of  mag- 
netic material,  and  also  upon  the  amount  of  mechanical  shock  to 
which  it  is  subjected.  There  is  a  more  or  less  gradual  weakening 
of  the  residual  magnetism,  rapid  at  first  and  approaching  stability  as 
time  elapses. 

768.  What  is  meant  by  "aging"  a  magnet? 

When  magnets  are  used  in  instruments,  it  is  quite  important  that 
they  remain  of  uniform  strength  for  a  long  while.  For  this  pur- 
pose different  methods  have'  been  devised  for  hastening  the  aging 
process  so  that  the  magnets  may  soon  arrive  at  the  stable  part  of 
their  life.  One  method  is  to  subject  the  magnet  to  severe  mechanical 
vibration  or  shock,  each  treatment  removing  some  of  the  original 
magnetization.  Another  process  is  to  put  the  magnet  in  a  steam 
bath.  A  process  used  somewhat  extensively  is  to  subject  the  mag- 
net to  the  influence  of  a  coil  carrying  an  alternating  current  which 
is  not  quite  strong  enough  to  reverse  the  original  magnetization.  A 
method  sometimes  employed  is  to  magnetize  the  steel  and  lay  it  away 
for  a  number  of  years.  When  a  magnet  is  properly  aged  it  will 
remain  quite  constant  for  a  long  time,  unless  it  is  subjected  to  abuse. 
Instruments  which  contain  permanent  magnets  should  therefore  be 
handled  carefully,  lest  the  magnets  become  weakened. 

769.  What  material  is  used  for  permanent  magnets? 

Hard  steel  is  most  common.  Tool  steel  is  good.  The  varieties 
known  as  tungsten  steel  and  chrome  steel  are  excellent  for  retaining 
magnetism.  After  the  steel  has  been  forged  or  otherwise  worked 
up  into  shape  it  should  be  hardened  and  tempered.  The  exact  tem- 
per depends  to  some  extent  upon  the  shape  and  dimensions  of  the 
magnet  and  upon  the  particular  kind  of  steel  employed.  Short  mag- 
nets should  be  harder  than  long  ones  for  the  best  retentivity. 


MAGNETISM.  193 

770.  How  are  permanent  magnets  charged? 

The  old  way  was  by  rubbing  them  against  another  permanent  mag- 
net or  against  a  lodestone.  The  modern  method  is  to  subject  them 
to  the  action  of  an  electromagnet.  In  some  cases  the  coil  wire  is 
placed  around  the  hardened  steel,  in  other  cases  the  steel  is  placed 
across  the  poles  of  a  strong  electromagnet.  It  is  sometimes  de- 
sirable to  strike  the  steel  gently  with  a  hammer  or  mallet,  so  as  to  set 
the  steel  into  vibration  in  order  that  the  molecules  may  more  easily 
rearrange  themselves  magnetically.  It  is  also  sometimes  desirable 
to  place  a  piece  of  soft  iron  across  the  poles  of  the  magnet  and  move 
this  up  and  down  or  from  the  bend  toward  the  poles. 

771.  What  is  a  compound  magnet? 

Magnets  are  sometimes  made  up  of  a  number  of  thin  pieces  rather 
than  of  one  thicker  piece  of  steel.  The  pieces  are  magnetized  sep- 
arately and  then  assembled.  Compound  or  laminated  magnets  are 


FIG.  771.— COMPOUND  HORSESHOE  MAGNET. 

generally  stronger  than  those  for  solid  metal,  probably  for  the  rea- 
son that  the  thick  steel  is  not  hardened  uniformly,  being  much  harder 
on  the  surface  than  inside,  and,  therefore^  does  not  become  so  uni- 
formly magnetized. 

772.  What  is  a  lodestone? 

The  lodestone  was  the  first  magnet  known.  It  is  a  piece  of  mag- 
netic iron  ore,  sometimes  called  magnetic  oxide,  or  magnetite,  com- 
posed of  three  parts  of  iron  and  four  parts  of  oxygen  and  represented 
by  the  chemical  formula  Fes  O*.  It  was  found  near  Magnesia  in  Asia 
Minor,  whence  came  the  name  magnet.  The  name  Lodestone  seems 
to  come  from  a  Saxon  word  meaning  to  lead,  from  the  use  of  the 
stone  as  a  compass. 

773.  How  does  lodestone  become  magnetised? 

It  is  probably  by  induction  from  the  magnetism  of  the  earth,  which 
in  turn  is  probably  caused  by  electric  currents  around  the  earth,  due 


194  ELECTRICAL   CATECHISM. 

to  some  action  of  the  sun.  It  is  known  that  the  earth  is  a  great 
magnet  with  one  pole  near  the  north  end  of  the  earth's  axis,  and  the 
other  pole  near  the  south  end  of  the  axis.  The  magnetic  north  pole 
is  at  about  80  deg.  latitude,  and  about  100  deg.  west  longitude,  being 
almost  directly  north  from  Minneapolis. 

774.  Is  the  earth's  magnetism  constant? 

It  varies  slightly  each  day  and  also  varies  from  a  quarter  to  a  third 
of  one  degree  at  different  times  during  the  year.  It  gradually 
changes  from  year  to  year  to  a  small  extent.  There  are  sudden 
variations  of  considerable  magnitude  that  are  called  magnetic  storms, 
which  interfere  with  telegraph  and  telephone  lines,  and  which  seem 
to  have  a  close  connection  with  the  aurora  borealis  and  with  sun 
spots.  The  apparent  position  of  the  north  is  greatly  disturbed  by 
reason  of  currents  of  electricity  in  the  earth  due  to  the  use  of  the 
ground  for  the  return  path  for  electric  railwavs. 

775.  What  is  the  compass? 

The  compass  is  a  piece  of  steel  magnetized  and  hung  so  as  to  be 
free  to  swing  on  a  horizontal  or  vertical  axis.  One  end  of  the 
movable  needle  will  point  toward  the  magnetic  north  pole  of  the 
earth  unless  disturbed  by  local  influences.  The  compass  is  used 


FIG.  775.-MAGNETIC  COMPASSES. 

largely  for  guiding  navigation,  and  also  land  travel  in  unknown  or 
unsettled  regions.  The  Chinese  are  said  to  have  been  acquainted 
with  the  compass  since  Hoang  Ti  in  2637  B.  C.  made  an  image  that 
always  pointed  toward  the  south  and  guided  their  armies.  The 
magnetic  needle  was  formerly  used,  to  a  large  extent,  in  electrical 
measuring  instruments,  but  has  now  been  abandoned  almost  com- 
pletely on  account  of  the  disturbances  from  the  electric  railways. 
The  needle  is  still  used  to  a  large  extent  in  land  surveying. 


MAGNETISM. 


195 


776.     W hat  is  the  inclination  compass? 

The  inclination  compass  or  dipping  needle  is  so  arranged  so  as  to 
move  in  a  vertical  direction.  If  a  magnetic  needle  were  placed  di- 
rectly over  one  of  the  magnetic  poles  of  the  earth,  it  would  point 


FIG.  776.-INCLINATION  COMPASS. 

neither  north  nor  south,  but  directly  downward.  At  the  equator  it 
would  tend  to  remain  horizontal,  and  at  intermediate  points  it  would 
tend  to  take  intermediate  positions. 

777.  What  is  meant  by  "H"  and  "V"  in  connection  with  terres- 
trial magnetism? 

The  magnetic  attraction  of  the  earth,  which  is  in  downward  direc- 
tion somewhere  between  the  vertical  and  horizontal,  may  be  thought 
of  as  made  of  two  elements  in  the  horizontal  and  vertical  directions, 

3C=  0.208 


FIG.  777.— COMPONENTS  OF  EARTH'S  MAGNETISM. 

and  these  are  called  the  horizontal  and  vertical  components,  which 
are  often  abbreviated  to  "H"  and  "V."  For  example,  in  New  York 
the  earth's  magnetism  has  a  force  of  0.6 1  dynes  in  a  direction  of  70° 
6'  below  the  horizontal.  This  force  may  be  resolved  into  two  com- 
ponents of  0.573  m  tne  vertical  direction  and  0.208  in  the  horizontal, 
as  suggested  in.the  figure. 


196 


ELECTRICAL   CATECHISM. 


778.  How  does  the  shape  of  a  magnet  affect  its  permanence? 

If  the  circuit  is  completely  closed  through  iron  or  steel,  there  i-3 
little  or  no  tendency  for  the  magnetism  to  leave  the  metal,  for  the 
lines  of  force  tend  to  take  the  easiest  path,  which  is  through  iron. 
But  if  the  circuit  consists  partly  of  air,  that  is,  if  there  is  a  gap  in  the 
circuit,  the  ends  seem  to  have  a  depolarizing  effect  as  if  one  part 
were  magnetized  more  strongly  than  other  parts,  and  so  reversed 
their  magnetism  and  used  them  as  return  paths.  If  a  complete  ring 
of  soft  iron  is  magnetized,  almost  all  of  the  lines  will  remain  in  the 
iron,  and  the  ring  retains  its  original  magnetism  after  the  magnetiz- 
ing current  is  removed.  But  if  the  ring  is  cut,  so  that  the  lines  of  force 
must  pass  through  a  small  air  space,  the  magnetism  quickly  disap- 
pears when  the  current  stops.  If  the  iron  is  laminated  or  slit  intc 
small  parts  near  the  ends,  the  magnetism  is  lost  more  quickly. 

779.  For  what  purposes  are  permanent  magnets  used? 
Permanent  magnets  are  used  in  the  form  of  needles  for  pointing  to 

the  magnetic  north  and  for  locating  magnetic  masses  such  as  beds  of 
iron  ore,  or  for  locating  iron  masses  in  places  difficult  of  access  ;  they 
are  used  to  furnish  fields  for  measuring  instruments,  for  small  mo- 
tors and  dynamos,  for  telephone  receivers,  for  polarized  telegraph 


Am.Elcc. 
FIG.  780.— MAGNETIZED  CASH  CARRIER. 

instruments,  for  polarized  bells  and  similar  apparatus,  for  separating 
magnetic  particles  from  non-magnetic,  for  attractive  toys  and  quack 
medical  apparatus,  for  lifting  iron,  for  attracting  small  particles 
such  as  tacks  and  screws.  Some  of  these  uses  are  also  applied  to 
electromagnets  and  some  are  really  elementary  motors,  and  will  be 
considered  in  that  connection. 

780.     How  can  a  magnet  be  used  for  locating  magnetic  masses? 

The  principle  is  illustrated  by  a  simple  device  sometimes  used  to 
locate  pneumatic  cash  carriers  which  sometimes  get  stuck  in  the 
tubes.  Pieces  of  clock  spring  are  magnetized  and  riveted  to  the  sides 
of  the  leather  carriers.  By  moving  a  compass  along  the  tube  a  car- 
rier can  be  located  by  the  attraction  and  repulsion  of  the  ends  of  the 
compass.  An  extension  of  the  idea  may  be  used  to  locate  masses  of 


MAGNETISM. 


197 


iron  ore  by  noting  the  deflections  of  a  compass  needle  which  is  moved 
to  different  points  near  the  supposed  bed  of  ore. 


FIG.  78lA.— MAGNETOMETER. 

781.  How  are  magnetic  surveys  made  for  locating  beds  of  iron 
ore? 

For  locating  bodies  of  magnetic  iron  ore,  sometimes  called  mag- 
netite or  Fe»  O*,  a  special  kind  of  compass  or  magnetometer  is  used, 
consisting  of  an  ordinary  compass  with  accurately  divided  circle 
and  arranged  with  a  controlling  magnet  sliding  on  an  arm.  This 
may  be  used  as  a  dipping  needle,  the  magnet  on  the  arm  being  ad- 
justed so  as  to  balance  the  horizontal  component  of  the  earth's  mag- 
netism, when  the  needle  is  affected  only  by  the  vertical  component. 
When  used  in  the  horizontal  position,  the  controlling  magnet  is  re- 
moved and  the  instrument  set  so  that  the  needle  points  to  zero ;  the 
magnet  is  then  replaced  and  the  deflection  of  the  needle  is  measured, 
giving  an  indication  of  the  strength  of  the  horizontal  component  of 
the  earth's  magnetism  at  that  point.  To  make  a  magnetic  survey  by 
either  method,  the  ground  is  staked  off  into  squares  about  30  feet  on 


FIG.  781s.— PLAT  OF  MAGNETIC  SURVEY. 

a  side  and  a  reading  is  taken  at  every  corner  of  every  square.    After 
the  whole  district  has  been  tested,  a  plot  of  the  ground  is  made  on 


198  ELECTRICAL  CATECHISM. 

which  the  points  giving  the  same  deflection  are  connected  by  lines 
which  will  be  found  to  give  closed  curves  somewhat  as  shown  in  the 
figure ;  one  of  the  curves  taken  with  the  instrument  in  the  horizontal 
position  will  be  found  to  extend  so  far  as  not  to  close  on  itself,  and 
the  intersection  of  this  curve  with  a  line  drawn  between  the  points  of 
maximum  and  of  minimum  deflection  will  indicate  the  center  of  the 
magnetic  mass.  The  curves  taken  with  the  instrument  in  the  hori- 
zontal position,  measuring  the  variations  in  the  horizontal  component 
of  the  earth's  magnetism,-  are  shown  by  the  solid  lines ;  those  taken 
by  measuring  the  inclination  or  the  vertical  component  are  shown 
by  the  dotted  lines  which  center  about  the  ore  mass.  These  methods 
are  used  in  Sweden  successfully. 

782.     How  is  magnetism  applied  to  hammers  and  screivdrivers? 

These  may  be  magnetized  with  one  pole  near  the  driving  end.  It 
is  often  very  convenient  to  have  a  screwdriver  magnetized  in  order 
to  hold  a  screw  on  the  end  and  locate  it  at  an  awkward  place.  Much 
time  may  be  saved  in  picking  up  tacks  and  holding  them  in  place  until 
set  by  the  hammer  if  the  hammer  itself  is  magnetized  so  that  it  will 


FIG.  782.— MAGNETIC  TACK  HAMMER. 

pick  up  the  tacks  and  place  them  properly.  It  is  not  necessary  to 
have  a  special  tool  for  such  purpose,  as  any  hammer  or  screwdriver 
can  be  magnetized  by  putting  it  upon  the  poles  of  an  electromagnet 
or  upon  one  pole  of  a  dynamo  or  motor. 

783.  How  are  magnets  used  in  connection  with  quack  medical 
apparatus  f 

Some  of  the  so-called  electric  hair  brushes  have  a  magnetized  steel 
wire  imbedded  in  the  back  and  this  attracts  and  repels  the  ends  of 
the  compass  needle  which  is  sold  with  the  brush  so  that  "the  odic 
force  can  always  be  tested  by  moving  the  brush  near  a  compass." 
Some  of  the  so-called  electric  corsets,  plasters  and  pads  are  of  the 
same  order.  "Electro-magnetic,"  "electro-chemical"  and  "magnetic" 
rings  for  the  cure  of  all  diseases  are  simply  steel  rings  magnetized. 
They  sometimes  are  the  occasion  of  real  cures  caused  by  the  faith 
of  the  wearer,  who  would  have  been  equally  benefited  by  wearing 
a  shark's  tooth  or  rabbit's  foot. 

784.  For  what  purposes  are  electromagnets  used? 

The  tendency  for  the  magnetic  lines  of  force  to  become  shorter  is 


MAGNETISM.  199 

used  for  many  purposes,  both  where  simply  attraction  is  desired  and 
where  attraction  and  a  resulting  motion  are  desired.  The  latter  is 
the  general  case  of  the  electric  motor.  In  some  cases  the  repulsion 
between  similar  poles  is  used  either  alone  or  in  connection  with  at- 
traction. In  a  few  cases  the  tendency  for  the  lines  to  spread  (see 
Nos.  712,  713)  is  used  alone.  The  relation  between  magnetism  and 
induced  E.M.F.  lies  at  the  bottom  of  the  dynamo,  motor,  trans- 
former; in  short,  the  most  important  developments  in  electrical  engi- 
neering. 

785.  For  zvhat  purposes  is  the  simple  attractive  force  of  the  mag- 
net used? 

Magnetic  clutches,  such  as  those  used  for  coupling  engines  and 
generators  in  the  Arnold  system  of  central  power  stations,  in  the 
regulating  device  on  the  new  Brush  arc  dynamos  and  certain  mag- 
netic devices  for  increasing  the  traction  between  moving  parts  for 
transportation  purposes ;  lifting  magnets,  such  as  used  for  handling 
iron  and  steel  plates,  for  clamps  to  hold  lamps  or  tools  in  place  on 
iron  surfaces,  separating  iron  particles  from  turnings  or  shop  sweep- 
ings or  from  the  useless  rock  of  iron  ores ;  release  magnets,  such  as 
used  for  holding  the  arms  of  rheostats  in  the  "on"  position ;  surgical 
magnets  for  locating  or  removing  iron  particles  in  the  eye  or  other 
parts  of  the  body. 

786.  For  what  purposes  are  magnets  used  to  secure  motion  as  a 
result  of  attraction? 

Elementary  electric  motors,  such  as  electric  bells,  telegraph  instru- 
ments, annunciators,  electric  tuning  forks,  arc  lamp  mechanisms,  in- 
duction coil  vibrators,  electric  organ  movements,  clocks,  electric 
locks,  door-openers,  burglar  alarms,  electric  measuring  and  regu- 
lating devices  "and  other  purposes  too  numerous  to  mention." 

787.  For  what  purposes  is  the  repellent  magnetic  action  used. 
In  connection  with  the  attractive  action,  it  is  used  in  "polarized" 

apparatus  such  as  the  vibrating  magneto  bell,  polarized  telegraph  in- 
struments, telephone  receivers,  certain  arc  lamp  regulators,  electric 
motors  and  measuring  instruments. 

788.  How  can  magnetic  attraction  be  used  for  driving  machinery? 
For  coupling  shafts  which  are  stationary  or  are  running  at  the 

same  speed,  a  simple  magnetic  clutch  consists  of  two  cast  steel  rings 
carried  on  steel  web  plates  which  are  bolted  to  hubs  on  the  shafts 
to  be  coupled.  One  of  the  rings,  called  the  field  ring,  has  an  annular 
slot  in  which  the  energizing  coil  is  secured ;  the  other,  called  the 
armature  or  keeper,  is  so  mounted  as  to  be  separated  from  the  field 


200 


ELECTRICAL  CATECHISM. 


ring",  when  no  current  passes  through  the  coil,  by  a  narrow  airgap. 
The  steel  web  plates  allow  the  two  rings  to  spring  together  when 
current  passes  through  the  coil  and  to  spring  apart  when  current 


FIG.   788A.-ENGINE  DRIVING   DYNAMOS   BY   MAGNETIC   CLUTCHES. 

'Armature 


FIG.  788B.— MAGNETIC   CLUTCH,   ACCELERATOR   AND   POLE   FACE. 

ceases.     A  small  current  causes  sufficient  attraction  between  the  two 
rings  to  hold  the  shafts  strongly  together. 

To  insure  gradual  and  smooth  starting  in  coupling  shafts  when 
one  is  stationary  and  the  other  running,  the  "  accelerator  "  has  a 
special  pole-face  having  intermeshing  polar  projections  separated  by 
a  filling  of  non-magnetic  material  such  as  babbitt  metal,  When  cur- 


MAGNETISM 


201 


rent  passes  through  the  coil,  the  magnetic  flux  through  the  armature 
is  not  uniform  as  in  the  simpler  clutch,  but  it  circulates  in  and  out 
from  one  pole  to  the  next  more  or  less  circumferentially  instead  of 
all  radially;  the  result  is  that  when  the  two  parts  are  revolving  at 
different  speeds,  electromotive  forces  are  induced  in  the  armature 
(see  Nos.  1113  and  1405^  which  cause  eddy  currents  to  flow  and 
exert  a  torque  (see  No.  1306)  which  varies  with  the  strength  of  the 
magnetism  and  the  difference  in  the  speeds  of  the  two  rings.  Either 
type  of  magnetic  clutch  can  be  operated  from  any  convenient  place 
by  simply  closing  or  opening  the  magnetizing  circuit. 


FIG.   789.— ELECTRO    MAGNET   FOR   LIFTING   IRON    PLATES. 

789.     For  what  are  lifting  magnets  used? 

These  are  found  useful  in  rolling  mills  and  similar  places  for 
handling  iron  plates,  being  found  cheaper,  safer  and  quicker  than 
the  chains  and  hooks  formerly  used.  Plates  weighing  as  high  as 
10,000  pounds  may  be  picked  up  with  ease  and  held  while  the  travel- 
ing crane  moves  them  to  the  place  desired.  The  figure  shows  their 
use  by  the  Illinois  Steel  Company. 


202  ELECTRICAL  CATECHISM. 

790.     What  is  an  eye  magnet? 

These  are  strong,  permanent  or  electromagnets  having  one  pole 
brought  to  a  point  which  is  very  strongly  magnetized  and  which, 
therefore,  will  strongly  attract  any  piece  of  iron  or  steel  near  which 


FIG.  790.— EYE  MAGNET. 

it  may  be  brought.  These  are  useful  in  removing  bits  of  iron  that 
may  become  imbedded  in  the  eye  or  in  other  places  difficult  of  access. 
Similar  magnets  have  been  successful  in  withdrawing  needles  buried 
in  the  flesh. 

791.  How  are  magnets  used  for  separating  iron  from  other  par- 
ticles? 

The  self-binders  for  harvesting  grain  formerly  used  iron  wire  for 
tying  the  bundles,  and  since  bits  of  wire  would  sometimes  get  mixed 
with  the  grain  during  threshing,  the  millers  found  difficulty  from  the 
iron  getting  into  their  machinery  and  flour.  To  obviate  this  diffi- 
culty they  placed  strong  magnets  around  the  hoppers  through  which 
the  grain  passed  into  the  mill  and  these  magnets  attracted  and  held 
the  wires.  For  separating  iron  from  brass  turnings  and  filings,  mag- 
nets are  arranged  around  the  inside  of  a  thin  brass  cylinder  over 
which  the  shop  refuse  is  allowed  to  fall ;  the  bits  of  iron  are  attracted 
by  the  magnets  and  are  held  by  the  cylinder  until  brushed  off,  while 
the  non-magnetic  particles  of  brass  fall  at  once  and  so  are  separated 
from  the  iron.  This  idea  has  been  developed  for  separating  iron  ore 
from  the  rock  in  which  it  usually  occurs.  Edison  has  developed  this 
process  to  a  commercial  stage. 

792.  How  does  Edison  concentrate  iron  ore  magnetically? 

The  ore  treated  is  a  magnetic  oxide  of  iron  held  in  about  three 
times  its  weight  of  easily  crushed  gangue  rock.  This  is  crushed  be- 
tween Tollers  and  then  allowed  to  fall  in  a  thin  sheet  in  front  of  a 
series  of  magnets  which  deflect  the  iron  particles,  but  allow  the  non- 
magnetic rock  to  fall  vertically.  A  thin  knife-edged  partition  board 
separates  the  two  falling  streams.  The  attracted  particles  are  dried 
and  ground  and  then  separated  again  from  the  rocky  material,  then 
treated  chemically  and  again  separated.  The  magnetic  separator  at 
Edison,  New  Jersey,  has  a  capacity  to  handle  300  tons  of  crushed 
rock  per  hour. 


MAGNETISM. 


203 


793.     How  are  magnetic  brakes  used? 

Brakes  operated  by  electromagnets  are  used  in  connection  with 


FIG.  792.— MAGNETIC  ORE  SEPARATOR. 

street  railway  and  other  motors  for  stopping  the  motion.  In  the 
case  of  electric  railways  the  electric  brake  consists  of  two  discs  fac- 
ing each  other  much  like  the  Arnold  magnetic  clutch;  one  disc  re- 
volves with  the  car  axle  while  the  other  is  stationary ;  when  current 
is  sent  through  the  magnetizing  coil,  the  two  are  attracted  together 
and  the  friction  quickly  stops  the  car.  In  some  electric  elevators  and 
with  some  motors  for  mill  work  where  it  is  necessary  to  stop  fre- 


FIG.  793.— ELECTROMAGNETIC  BRAKE  ON  MOTOR. 

quently  and  at  a  definite  point,  electromagnets  are  sometimes  ar- 
ranged to  tighten  or  release  a  band  brake  which  clamps  a  pulley  on 
the  motor  shaft,  as  indicated  in  the  illustration  of  an  electromagnetic 
brake  on  a  mill  motor. 


CHAPTER  VII. 


ELECTRICAL  MEASURING  INSTRUMENTS. 

800.  Upon  what  principles  are  electrical  measuring  instruments 
made? 

Every  effect  of  electricity  has  a  quantitative  relation  to  the  cause, 
and  nearly  every  effect  has  been  made  the  basis  of  measuring  instru- 
ments. The  effects  most  commonly  used  are  the  static,  heating, 
chemical  and  magnetic. 

801.  How  is  the  electro-static  effect  used  for  electrical  measure- 
ment? 

A  number  of  instruments  are  based  upon  the  fact  that  two  con- 
ductors attract  one  another  when  any  difference  of  potential  or  elec- 
tric pressure  exists  between  them.  If  one  is  delicately  suspended  so 
as  to  be  free  to  move,  it  will  approach  the  other.  This  is  developed 
in  the  electro-static  voltmeters  of  Lord  Kelvin  (Sir  William  Thom- 
son) and  others,  also  in  the  electro-static  ground  detectors  of  the 
Stanley  and  the  General  Electric  companies. 

Electro-static  voltmeters  were  considered  in  Nos.  140  to  147.  Cur- 
rent may  be  measured  by  fall  of  potential  through  a  known  resist- 
ance by  means  of  a  static  voltmeter,  although  such  instruments  are 
not  usually  suitable  for  measuring  small  voltages.  Electro-static  in- 
struments have  been  made  for  measuring  watts,  but  "have  not  proved 
sufficiently  satisfactory  to  obtain  a  hold  upon  the  market. 

802.  De-scribe  and  explain  the  electrostatic  voltmeter. 

The  Kelvin  voltmeters  suitable  for  direct  or  alternating  pressures 
from  40  to  100,000  volts,  and  the  quadrant  electrometer  suitable 
for  direct  pressures  as  low  as  o.ooi  volt,  were  discussed  in  Nos.  '142 
to  147.  The  Westinghouse  company  has  developed  a  line  of  electro- 
static voltmeters  for  pressures  from  2500  to  120,000  volts,  using  con- 
densers in  series.  In  the  figure,  Mj  and  M2  are  movable  condenser 
elements  consisting  of  hollow  spherical  end  cylinders  supported  on 
a  steel  ball  bearing  mounted  in  polished  jewels ;  Ba  and  B2  are  curved 
metallic  sheets  forming  the  opposite  plates  of  condensers  which 
Mj  and  M2  approach  as  they  rotate;  Q  and  C2  are  pairs  of  plates 
of  condensers  in  series,  being  connected  on  one  side  to  the  instrument 


INSTRUMENTS. 


205 


terminals  T\  and  T2  and  on  their  other  sides  to  the  inner  condenser 
plates  Bx  and  B2.  The  rotation  of  Mj  and  M2  is  opposed  by  con- 
trolling springs,  the  position  of  equilibrium  where  the  attraction 


FIG.  802.— WESTINGHOUSE   ELECTROSTATIC   VOLTMETER. 

between  the  fixed  plates  B  and  the  moving  cylinders  M  is  balanced 
by  the  springs  being  indicated  by  a  pointer  P  moving  along  the  scale 
S.  The  containing  case  is  filled  with  oil  which  buoys  up  the  moving 
element,  acts  as  a  damper,  maintains  insulation  and  increases  the 
capacity. 

803.     Describe  and  explain  the  electro-static  ground  detector. 

One  simple  form  is  shown  in  the  figure.  It  is  a  sort  of  differential 
static  voltmeter  having  one  movable  vane  and  two  stationary  pairs 
of  vanes.  The  movable  vane  is  electrically  connected  with  the  ground ; 


FIG.  803— ELECTROSTATIC  GROUND  DETECTOR. 


206  ELECTRICAL   CATECHISM. 

one  pair  is  connected  to  the  side  of  the  circuit  to  be  tested  and  the 
second  pair  to  the  other  side.  When  both  sides  of  the  circuit  are 
highly  insulated  so  as  to  be  free  from  "grounds,"  the  needle  is  at- 
tracted equally  by  both  pairs  of  vanes  and  so  remains  in  the  neutral 
position.  Now,  suppqse  that  the  insulation  of  one  side  becomes  im- 
paired so  that  it  is  more  or  less  grounded ;  the  pair  of  vanes  connected 
with  that  side  now  attracts  the  needle  less  strongly  than  before,  be- 
cause the  difference  of  pressure  between  them  is  less  than  before; 
the  attraction  between  the  needle  and  the  other  pair  of  vanes  is  not 
less,  but  rather  more,  than  before,  because  the  opposite  wire  has  ap- 
proached more  nearly  the  potential  of  the  ground  and  of  the  grounded 
needle.  The  needle  therefore  moves  away  from  the  pair  of  vanes 
connected  with  the  wire  that  is  grounded.  The  instrument  shown 
is  that  of  the  Stanley  Company,  that  of  the  General  Electric  Com- 
pany being  similar  in  principle.  For  use  on  circuits  of  more  than 
10,000  volts,  the  stationary  vanes  are  not  connected  directly  with  the 
wires,  but  are  charged  inductively  by  auxiliary  vanes  which  are  in- 
sulated from  the  instrument  and  are  connected  with  the  circuit. 

In  the  General  Electric  static  detectors,  two  adjacent  quadrants 
are  connected  with  the  ground,  the  other  two  being  connected  re- 
spectively with  the  two  sides  of  the  circuit.  The  moving  vane  is 
charged  inductively  and  is  equally  attracted  by  both  charged  quad- 
rants when  the  lines  are  free  from  grounds,  consequently  remaining 
in  the  zero  or  neutral  position  except  when  a  "ground"  exists. 

804.  How  can  the  heating  effect  of  the  current   be   used   for 
measurement? 

Since  every  conductor  has  more  or  less  resistance,  it  causes  a  drop 
in  pressure  of  any  current  passing,  the  drop  being  equal  to  the 
product  of  current  by  the  resistance.  This  involves  the  absorption 
of  energy  equal  to  the  product  of  current  by  drop.  The  energy 
thus  absorbed  appears  as  heat,  which  raises  the  temperature  of  the 
conductor.  The  amount  of  heat  developed  in  a  given  time  may  be 
measured  by  an  instrument  called  a  calorimeter,  from  which  the  cur- 
rent may  be  calculated  if  it  has  been  constant.  This  method  was  a 
favorite  one  when  alternating  currents  first  came  into  use,  but  it  is 
inconvenient  and  is  rarely  used  at  present.  (See  Nos.  414  to  421.) 

805.  In  what  other  way  may  the  heating  effect  of  the  current  be 
used  for  measuring? 

The  energy  absorbed  by  the  resistance  of  the  wire  raises  the  tem- 
perature of  the  conductor  above  that  of  the  surrounding  objects.  The 
rate  at  which  heat  is  interchanged  between  objects  varies  directly  as 


INSTRUMENTS. 


207 


the  difference  of  temperature  between  them ;  therefore,  heat  is  given 
off  to  surrounding  objects  more  and  more  rapidly  as  the  temperature 
of  the  wire  increases.  A  point  will  soon  be  reached  where  the  heat 
is  given  off  as  fast  as  it  is  developed  in  the  conductor,  and  then  the 
temperature  becomes  constant.  At  this  point  the  conductor  and  the 
surrounding  objects  have  expanded  a  definite  ratio  larger  than  their 
original  size.  The  expansion  of  either  the  conductor  or  the  sur- 
rounding objects  may  be  used  to  indicate  the  temperature,  and  there- 
fore indirectly  the  current  which  caused  it. 

806.  How  is  the  expansion  of  the  conductor  used  to  measure  cur- 
rent? 

Expansion  of  a  conductor  is  suitable  for  measuring  direct  or 
alternating  current  or  pressure,  since  the  current  through  a  con- 
ductor is  proportional  to  the  voltage  between  its  terminals.  The 


FIG.  806.— CARUEW   AND    WHITNEY   HOT-WIRE   VOLTMETERS. 


increase  in  length  of  the  conductor  being  small,  it  is  multiplied  in 
various  ways.     In  the  Cardew  voltmeter,  formerly  the  best  portable 


203 


ELECTRICAL  CATECHISM. 


instrument  for  measuring  alternating  E.M.F.s,  the  expansion  .of  a 
fine  wire  running  over  a  series  of  pulleys  was  transmitted  through 
a  cord  to  multiplying  gearing  which  moved  a  pointer  across  a  scale. 
In  a  recent  Whitney  instrument,  current  passing  through  a  fine  wire 
a  of  high  resistance  and  low  temperature  coefficient,  causes  it  to  heat 
and  lengthen,  thus  turning  the  pulley  d  and  the  attached  arm  g,  to 
whose  bifurcated  end  is  attached  a  silk  fiber  wound  around  a  shaft  h 
which  carries  a  long  pointer  i. 

807.  Upon  what  principles  do  the  Hartman-Braun  instruments 
work? 

These  instruments  are  somewhat  similar  to  those  made  for  some 
time  by  Queen  &  Company.  The  instruments  involve  several  inter- 
esting peculiarities  which  improve  their  operation  over  that  of  pre- 
vious types  and  the  method  of  adapting  the  hot  wire  principle  to  am- 
meter use  is  also  interesting.  The  figure  shows  a  general  outline  of 


FIG.  807.-HOT-WIRE  INSTRUMENT. 

the  parts  and  their  connections.  The  hot  wire  proper,  shown  at 
A  A,  is  made  of  platinum  silver  about  6  or  7  ins.  long,  stretched  be- 
tween two  terminals,  T'  and  T",  until  it  is  almost  taut.  Near  the 
center  of  this  wire  there  is  attached  another  wire  of  phosphor  bronze, 
B  B,  running  to  a  third  terminal,  T'",  at  the  lower  part  of  the  in- 
strument. Branching  from  this  phosphor-bronze  wire  is  a  cocoon 
fiber,  C,  which  is  looped  around  the  jewel-mounted  steel  spindle 
carrying  the  needle,  and  terminates  in  the  spring,  S,  which  maintains 
it  taut.  The  whole  arrangement  of  the  fibre  and  wires  is  thus  sub- 
jected to  tension  and  any  slacking  or  sag  of  the  hot  wire  at  the  top 
is  immediately  taken  up  by  the  steel  spring  and  transmitted  by  the 
motion  of  the  cocoon  fibre  to  the  pointer.  As  is  well  known,  when 
a  wire  is  stretched  nearly  straight  between  two  points  the  slightest 
change  of  its  length  causes  a  great  change  in  the  amount  of  slack. 
The  arrangement  of  the  platinum  silver  and  the  phosphor-bronze 


INSTRUMENTS.  209 

wires  thus  obtains  a  double  multiplication  of  the  deflection  in  this 
way,  a  third  multiplication  being  given  by  the  ratio  of  the  length  of 
the  pointer  to  the  diameter  of  the  small  spindle  on  which  the  cocoon 
fibre  is  coiled.  Thus  a  very  slight  change  in  length  of  the  wire,  A  A, 
causes  a  large  movement  of  the  pointer.  The  whole  hot  wire  move- 
ment is  mounted  on  a  metal  compensation  plate  made  from  an  alloy 
whose  temperature  coefficient  is  the  same  as  that  of  the  measuring- 
wire.  To  deflect  the  pointer  of  the  voltmeter  over  the  full  scale  a  cur- 
rent of  .2  amp.  is  required.  For  voltmeter  work  a  series  resistance  or 
multiplier  of  constantin  is  connected  in  series  with  the  hot  wire.  For 
ammeter  work  the  hot  wire  is  tapped  at  several  points  by  thin  silver 
foil  strips  which  divide  it  into  sections,  these  sections  being  placed  in 
parallel  with  each  other.  By  this  means  4  amp.  or  5  amp.  may  be 
sent  through  the  wire  with  a  drop  of  potential  not  exceeding  ^  volt 
(the  ammeter  hot  wire  being  thicker  than  the  voltmeter  hot  wire). 
For  higher  currents  a  constantin  shunt  is  used.  A  magnetic  damp- 
ing arrangment  is  provided  to  prevent  oscillations. 

808.  How  is  the  expansion  of  objects  near  a  conductor  used  for 
measuring  the  current? 

Several  plans  have  been  proposed,  although  only  one  seems  to  be 
a  commercial  success.  One  plan  proposed  by  Forbes  is  to  arrange 
the  conductor  as  a  coil  below  a  small  windmill  which  will  be  rotated 
by  the  air  put  in  circulation  by  the  heating  of  the  conductor.  The 
speed  of  the  windmill,  or  the  number  of  turns  it  makes  in  a  given 
time,  registers  the  amount  of  the  current.  Others  have  tried  placing 
the  conductor  around  a  thermometer  whose  rise  of  temperature 
would  thus  measure  indirectly  the  current  in  the  conductor.  One 
form  of  this,  known  as  the  Wright  maximum  demand  meter,  is  com- 
ing into  extensive  use  for  determining  the  largest  current  used  at 
any  time  by  a  consumer. 

809.  Explain  the  Wright  demand  meter. 

It  consists  of  a  glass  tube  with  two  bulbs,  around  one  of  which  the 
conductor  is  wrapped,  the  tube  being  partly  filled  with  a  liquid,  as 
shown  in  the  accompanying  illustrations.  The  passage  of  current 
heats  the  air  in  the  left  bulb  and  the  expansion  of  the  air  inside  forces 
more  or  less  of  the  liquid  into  the  bulb  at  the  right,  and  into  the 
graduated  overflow  tube,  from  which  it  cannot  be  removed  except 
by  tipping  up  the  meter.  The  amount  of  liquid  that  flows  over  de- 
pends upon  the  amount  of  expansion  of  the  air,  which  depends  upon 
the  strength  of  the  current.  Any  larger  current  will  send  more  liquid 
into  the  overflow  or  indicating  tube,  but  a  smaller  current  will  not. 


210  ELECTRICAL   CATECHISM. 

The  instrument  thus  indicates  the  largest  amount  of  current  used  at 
any  time,  which  gives  the  station  manager  a  basis  for  estimating  the 


FIG.  809.— MAXIMUM  DEMAND  METER. 

discount  to  be  allowed  from  the  bill  as  calculated  from  the  regular 
meter. 

810.     Is  any  other  heating  effect  used  for  measuring  the  current? 

The  fact  that  the  resistance  of  a  conductor  changes  with  the  tem- 
perature has  been  used  for  purposes  of  measurement,  notably  in  the 
Howell  lamp  indicator  or  voltmeter,  formerly  made  by  the  Edison 
Company. 

8n.  Has  the  change  of  resistance  with  temperature  been  applied 
to  measuring  instruments  f 

This  effect  has  been  used  in  the  Howell  instruments  for  measuring 
voltage,  and  in  a  number  of  instruments  for  measuring  temperature 
such  as  electrical  thermometers,  and  in  instruments  for  measuring 
radiant  heat  such  as  the  bolometer. 

Si  2.     What  is  the  principle  of  an  electric  thermometer  f 
A  conductor  having  a  high  melting  temperature,  such  as  carbon 
or  platinum,  is  inclosed  in  some  refractory  substance  such  as  por- 


INSTRUMENTS.  211 

cekin  and  exposed  to  the  high  temperature  which  is  to  be  measured. 
Knowing  the  temperature  coefficient  of  the  conductor  and  the  change 
in  its  resistance,  the  temperature  may  be  calculated.  Such  an  in- 
strument is  often  called  a  pyrometer.  For  use  at  high  temperatures, 
carbon  must  be  sealed  away  from  air  to  prevent  its  combustion.  For 
measuring  very  small  differences  of  temperature,  the  bolometer  has 
been  found  wonderfully  sensitive. 

813.  What  is  the  bolometer? 

The  common  form  of  bolometer  consists  of  two  similar  circuits 
containing  conductors  of  a  material  whose  resistance  changes  rapidly 
with  changes  of  temperature;  these  are  arranged  so  that  one  may 
be  exposed  to  the  source  of  heat  under  investigation,  while  the  other 
is  protected  from  it.  The  two  circuits  are  connected  respectively  to 
the  two  coils  of  a  differential  galvanometer  which  are  wound  or  con- 
nected oppositely  so  that  the  needle  is  only  affected  by  the  difference 
between  the  currents  in  the  two  coils.  When  both  circuits  are  con- 
nected to  the  same  battery  and  both  are  at  the  same  temperature, 
equal  currents  flow  through  the  two  coils  and  the  needle  is  not  af- 
fected; but  if  the  temperature  of  one  is  different  from  that  of  the 
other,  their  resistances  vary  and  the  currents  no  longer  balance, 
thus  causing  a  deflection  of  the  needle.  Such  instruments  have  been 
made  so  sensitive  as  to  be  affected  by  the  heat  from  a  candle  a  mile 
distant  and  even  to  show  the  heat  from  the  distant  stars. 

814.  Upon  what  principle  was  the  Howell  lamp  indicator  made? 

It  depended  upon  the  fact  that  the  resistance  of  most  metals  in- 
creases with  rise  of  temperature,  while  that  of  carbon  decreases. 
Two  circuits  are  arranged  so  that  when  equal  or  proportional  cur- 
rents flow  in  each,  one  neutralizes  the  effect  of  the  other  upon  a 
galvanometer  or  other  current  indicator.  One  circuit  contains  a 
carbon  resistance  consisting  of  a  special  incandescent  lamp ;  the  other 
contains  a  long  fine  wire  of  iron  or  some  other  such  metal  whose  re- 
sistance increases  rapidly  as  its  temperature  increases.  These  two 
circuits  are  coupled  in  multiple  and  are  connected  across  the  circuit, 
whose  voltage  is  to  be  kept  constant.  If  the  pressure  increases,  more 
current  flows  through  each  circuit  and  thus  raises  the  temperature 
of  each ;  this  increases  the  resistance  of  one  and  decreases  that  of  the 
other,  so  that  the  two  currents  are  no  longer  equal  or  proportional 
as  before,  so  that  one  affects  the  galvanometer  and  deflects  the 
pointer  in  one  direction.  If  the  voltage  becomes  lower  than  the 
standard  for  which  the  instrument  was  set,  the  resistances  change  in 
the  opposite  direction  and  the  pointer  is  moved  accordingly.  This 


212 


ELECTRICAL   CATECHISM. 


instrument  does  not  measure  the  voltage,  but  simply  shows  whether 
it  is  above  or  below  the  standard.    It  is  not  made  now. 

815.     What  is  the  electro-chemical  effect  as  used  for  measuring 
current? 

When  current  passes  through  any  liquid  conductor  (unless  it  be 
an  elementary  substance,  such  as  mercury)  the  liquid  is  decomposed, 
part  going  to  the  "anode,"  the  conductor  where  the  current  enters 
the  liquid,  and  part  going  to  the  "cathode,"  where  the  current  leaves 
the  liquid.  The  anode  is  dissolved  to  some  extent.  When  used  for 
measuring,  it  is  customary  to  have  as  the  liquid  a  solution  of  some 
salt  of  the  same  metal  that  is  used  for 
the  anode.  (See  Nos.  644  to  652.)  In 
the  standard  form,  pure  silver  is  used 
for  the  anode,  a  solution  of  nitrate  of 
silver  for  the  electrolyte  (the  liquid), 
and  a  platinum  bowl  for  the  cathode. 
A  current  of  i  amp.  deposits  0.001118 
grm.  of  silver  per  second  upon  the 
cathode.  Such  an  apparatus  is  called 
a  voltameter. 


FIG.  816.— EDISON   AND   BASTIAN   CHEMICAL   METERS. 

8 1 6.     Is  the  voltameter  used  as  a  commercial  instrument ? 

The  chemical  ampere-hour  meter  developed  by  Edison  was  used 
extensively  for  a  number  of  years  until  the  development  of  other 
meters  less  expensive  to  read  and  maintain.  About  i-iooo  of  the 
whole  current  passed  through  the  voltameters  consisting  of  two  zinc 
plates  in  a  zinc  sulphate  solution,  the  two  bottles  being  in  series  (as 


INSTRUMENTS.  £13 

a  check)  and  connected  around  a  German  silver  "  shunt  "  which  took 
the  main  current ;  the  weight  lost  by  anodes  measured  ampere-hours. 
The  cut  shows  a  meter  for  ampere-hours  on  each  side  of  a  three-wire 
system.  An  incandescent  lamp  operated  by  a  thermostatic  switch 
(on  the  same  principle  as  recent  blinking  sign  lights),  became  lighted 
whenever  the  temperature  became  low  enough  to  endanger  freezing. 
The  more  recent  Bastian  meter  sends  the  entire  current  through 
nickel  electrodes  in  an  alkaline  solution,  the  ampere-hours  (or  the 
watt-hours  when  multiplied  by  the  average  voltage)  being  indicated 
by  the  difference  in  the  level  of  the  liquid  which  is  decomposed  into 
gases  which  escape. 

817.  Is  the  voltameter  an  accurate  instrument ? 

It  is  in  the  hands  of  skilled  operators.  In  fact,  the  voltameter  has 
been  adopted  by  the  British  and  by  the  United  States  governments 
as  the  standard  means  for  calibrating  instruments  for  measuring 
currents,  but  is  almost  never  used.  (See  No.  227.) 

818.  Does  the  chemical  meter  measure  the  zvatt-hours  used  by  a 
consumer? 

It  measures  ampere-hours,  which  may  be  multiplied  by  the  average 
voltage  so  as  to  indicate  watt-hours.  The  meter  is  commonly  cali- 
brated to  show  kilowatt-hours. 

819.  Can  the  voltameter  be  used  as  a  voltmeter  for  measuring 
pressure? 

Not  in  its  ordinary  form.  It  has  been  proposed  to  make  one  with 
a  very  high  resistance,  in  which  case  the  current  would  be  propor- 
tional to  the  pressure.  The  change  of  weight  of  the  electrodes  might 
then  be  taken  to  determine  the  average  pressure.  This  has  not  been 
very  successful. 

820.  Can  the  voltameter  be  used  as  an  ammeter? 

Not  directly.  The  change  of  weight  during  a  given  time  measures 
the  average  current  during  that  time,  so  that  it  may  be  considered 
as  an  ammeter  when  the  current  remains  constant.  It  is  not  con- 
venient for  such  purposes  and  is  only  used  for  scientific  tests. 

821.  Is  the  voltameter  suitable  for  use  as  a  meter  for  alternating 
currents? 

No.  Since  the  alternating  current  goes  first  in  one  direction  and 
then  in  the  other,  whatever  effect  might  be  produced  by  one  alter- 
nation would  be  reversed  by  the  next,  except  that  in  voltameters  in 
which  gases  are  produced,  some  of  the  gas  might  escape  between  re- 
versals. 


21.4  ELECTRICAL  CATECHISM. 

822.  How  is  the  magnetic  effect  of  the  current  used  in  electrical 
measurement? 

Some  instruments  are  based  upon  the  mutual  attraction  or  repul- 
sion between  wires  carrying  current  due  to  the  magnetic  field  of 
force  surrounding  the  wires ;  some  use  the  reaction  between  the  mag- 
netic field  about  the  wire  and  an  independent  field  due  to  some  other 
source;  some  depend  upon  the  attraction  between  the  wire  and  the 
field  induced  by  it  in  a  soft  iron  core. 

823.  Is  it  necessary  to  have  iron  in  a  magnetic  circuit ? 

Not  always.  In  some  cases  it  is  better  to  have  none,  as  in  the  case 
of  certain  instruments  for  measuring  alternating  currents,  such  as 
the  Siemens  electrodynamometer,  the  Weston  alternating-current 
voltmeter  or  the  Thomson  recording  wattmeter. 

824.  What  is  there  in  common  between  measuring  instruments 
and  electric  motors? 

Nearly  every  instrument  for  measuring  current,  pressure  or  energy 
is  really  a  motor  in  which  the  armature  can  rotate  through  only  part 
of  a  revolution.  A  brief  discussion  of  certain  instruments  may  there- 
fore properly  introduce  the  study  of  the  motor. 

825.  What  is  the  electrodynamometer? 

It  is  an  instrument  for  measuring  amperes,  volts  or  watts.  The 
Siemens  electrodynamometer  consists  of  two  coils  at  right  angles 
to  each  other,  one  coil  being  stationary  while  the  other  is  free  to 
rotate.  The  figure  shows  a  common  form,  in  which  the  stationary 
coil  is  inside,  the  movable  coil  being  in  the  form  of  a  rectangle  outside 


FIG.  82').— ELECTRODYNAMOMETERS. 

A -Turning  knob  B=  Spring  c-  Moving  coil 

I)  =  Stationary   coil  E=  Mercury  cups 

the  other  and  hung  from  a  thread  inside  the  spiral  spring.    Electrical 
connection  with  the  movable  coil  is  made  through  mercury  cups  into 


INSTRUMENTS.  215 

which  the  ends  of  the  coil  dip.  When  currents  are  sent  through  the 
two  coils  the  magnetic  stresses  tend  to  move  the  two  coils  so  as  to 
become  parallel,  and  the  movable  coil  then  begins  to  rotate  to  a  posi- 
tion at  right  angles  to  that  shown  in  the  figure.  The  movable  coil  is 
then  brought  back  to  its  original  position  by  rotating  the  torsion  head 
to  which  one  end  of  the  spring  is  attached.  The  amount  of  twist 
given  the  spring  by  the  torsion  head  in  order  to  bring  the  coil  back 
to  zero  is  a  measure  of  the  current  in  the  two  coils. 

826.  How  can  the  electrodynamometer  be  used  as  a  voltmeter? 
By  having  both  coils  made  of  a  large  number  of  turns  of  fine  wire 

the  instrument  is  sensitive  to  small  currents.  Then  by  connecting  a 
high  resistance  in  series  with  the  instrument  it  may  be  connected 
across  the  terminals  of  the  circuit  whose  pressure  is  to  be  measured. 
The  electrodynamometer  really  measures  the  current  passing  through 
it,  but  by  Ohm's  law  this  is  proportional  to  the  pressure  or  E.M.F. 
at  its  terminals,  and  the  force  is  therefore  a  measure  of  the  E.M.F. 

827.  How  can  the  electrodynamometer  be  used  for  measuring 
watts? 

The  magnetic  force  of  each  coil  is  proportional  to  the  current 
through  it,  and  the  force  tending  to  rotate  the  movable  coil  equals 
the  product  of  the  magnetic  forces  of  the  two  coils.  If,  therefore, 
one  coil  is  suitable  for  carrying  the  main  current,  and  the  other  con- 
sists of  many  turns  of  fine  wire  with  high  resistance  suitable  for  con- 
necting across  the  circuit,  the  force  between  the  two  coils  will  be 
proportional  to  the  product  of  current  by  volts,  and  therefore  meas- 
ures the  watts.  The  instrument  is  then  called  a  watt-meter. 

828.  What  are  the  advantages  of  the  electrodynamometer? 

It  is  suitable  for  measuring  currents  either  direct  or  alternating, 
and  its  calibration  is  independent  of  the  frequency  or  form  of  the 
alternating  current.  Having  no  iron  or  permanent  magnets,  its  re- 
liability and  permanency  depend  upon  the  spring,  which  experience 
shows  does  not  change  from  year  to  year. 

829.  What  are  its  disadvantages? 

The  instrument  must  be  kept  carefully  leveled,  and  must  be  kept 
free  from  outside  magnetic  influences.  It  is  not  direct  reading,  but 
the  torsion  head  must  be  adjusted  so  that  the  pointer  comes  back  to 
zero  each  time  before  a  reading  can  be  made.  It  therefore  requires 
some  time  and  careful  handling,  and  is  of  little  use  when  the  current 
fluctuates  rapidly. 


216 


ELECTRICAL  'CATECHISM. 


830.  Are  there  other  instruments  based  on  the  electrodynamic 
effect  of  the  current? 

Besides  the  Siemens  electrodynamometer,  there  are  other  forms  by 
Kohlrausch,  Weber,  Thomson  and  others,  which  are  also  zero  instru- 
ments that  must  be  adjusted  for  each  reading.  Some  others,  such  as 
the  Weston  alternating-current  voltmeter,  are  direct  reading,  the 
coils  being  so  shaped  that  the  movable  coil  can  rotate  through  a  wide 
angle  while  a  pointer  attached  to  it  travels  across  a  scale  and  shows 
the  voltage  or  E.M.F.  at  the  terminals  of  the  instrument.  In  the 
Bristol  recording  voltmeter  the  two  coils  are  parallel  and  repel  one 
another,  the  movable  coil  carrying  a  pointer  that  moves  across  a 
scale. 

831.  How  can  one  tell  whether  a  wire  will  be  attracted  or  repelled 
in  a  given  case? 

The  general  rule  is  that  (a)  the  lines  of  force  are  considered  as 
coming  from  the  north  pole  and  going  into  the  south  pole  (b),  the 
lines  of  force  around  the  current  are  in  the  direction  of  motion  of 
the  hands  of  a  clock  when  one  looks  in  the  direction  in  which  the 
current  is  considered  as  flowing,  (c)  the  effect  of  bringing  two 
magnetic  forces  together  is  to  strengthen  the  field  where  the  two 
forces  are  in  the  same  direction  and  to  weaken  it  where  they  are  in 
opposite  directions,  and  (d)  the  lines  tend  to  separate  so  as  to 
make  the  field  of  uniform  strength.  It  is  easier  for  some  to  remem- 
ber the  "rule  of  thumb"  for  the  motor  or  dynamo. 

,    832.     What  is  a  rule  of  thumb  for  remembering  the  direction  of 
pull  on  a  wire? 

It  is  similar  to  that  for  the  E.M.F.  of  a  dynamo,  except  that  the  left 
hand  is  taken  instead  of  the  right  hand.  Spread  out  the  thumb  and 


fAOTOR 

FIG.  832.— MNEMONIC  RULE. 


first  and  second  fingers,  so  that  all  stand  at  right  angles  to  one  an- 
other. Let  the  forefinger  represent  the  direction  of  the  lines  of  force, 
the  center  finger  the  direction  of  the  current,  and  the  thumb  the 


INSTRUMENTS.  21? 

direction  of  the  motion.  Then,  if  one  knows  any  two,  he  can  easily 
get  the  direction  of  the  other.  The  fingers  of  the  right  hand  may 
be  remembered  as  for  the  dynamo  and  the  left  for  the  motor,  since 
the  right  hand  usually  comes  before  the  left  and  the  dynamo  must 
work  before  the  motor  will  start.  The  figure  shows  it  all. 

833.  What  instruments  are  developed  from  the  force  between  a 
current  and  a  magnetic  field? 

The  large  class  of  instruments  using  permanent  magnets,  also 
many  using  electromagnets.  Many  of  these  are  simply  portable 
forms  of  d'Arsonval  galvanometers. 

834.  What  is  the  d'Arsonval  galvanometer? 

It  consists  of  a  coil  suspended  between  the  poles  of  a  magnet  so 
that  it  may  rotate  through  a  small  angle  when  current  passes.  The 
figures  show  a  simple  form  arranged  for  fastening  to  a  wall  and  a 
more  sensitive  one  for  fine  work.  The  coil  is  suspended  by  a  small 
flat  wire  which  forms  one  terminal  of  the  coil,  the  connection  with 
the  other  terminal  being  made  through  a  very  weak  spiral  spring 
below.  The  coil  carries  a  pointer  that  moves  across  the  scale  for 


MAGNET 


FIG.  834.— D'ARSONVAL  GALVANOMETERS. 

reading  large  deflections.  A  mirror  also  fastened  to  the  coil  allows 
the  use  of  a  telescope  and  scale  for  reading  very  small  deflections  of 
the  coil  caused  by  correspondingly  small  currents.  The  cylindrical 
piece  of  iron  in  the  center  of  the  coil  in  the  simpler  instrument  is  sta- 
tionary, and  simply  serves  to  better  the  magnetic  circuit,  and  so 
make  the  field  stronger  than  it  would  be  if  the  whole  space  between 
the  poles  consisted  of  non-magnetic  material,  such  as  air. 


218 


ELECTRICAL   CATECHISM. 


835.  What  instruments  are  developments  of  the  d'Arsonval  gal- 
vanometer? 

Those  best  known  are  the  Weston  ammeters  and  voltmeters,  in 
which  the  moving  coil  is  supported  between  jeweled  bearings,  the 
current  being  taken  into  and  out  of  the  coil  by  means  of  delicate  hair 
springs,  which  also  serve  as  controlling  springs  to  balance  the  de- 
flecting force  of  the  current  in  the  coil.  The  "heart"  of  the  instru- 
ment is  shown  in  Fig.  83 5 A,  in  which  parts  of  the  magnet  and  pole 


FIG.  835.-AMMETER. 


FIG.  835A.— WORKING  PARTS  OF  WESTON 
INSTRUMENT. 


piece  are  cut  away  to  show  the  moving  coil,  the  central  soft  iron  core, 
the  two  springs  and  the  pivots. 

836.  Do  not  the  currents  in  the  two  sides  of  the  coil  go  in  opposite 
directions  and  so  oppose  each  other? 

The  currents  are  in  opposite  directions,  going  up  on  one  side  and 
down  on  the  other.  The  result  is  that  one  side  is  pulled  toward  the 
observer,  while  the  other  is  pushed  away.  But  both  pull  and  push 
tend  to  rotate  the  coil  in  the  same  direction  about  its  center,  and  so 
work  together  to  give  the  deflection. 

837.  What  is  the  general  principle  upon  ivhich  electromagnetic 
instruments  are  based  ? 

Electromagnetic  instruments,  which  include  nearly  all  ammeters 
and  voltmeters,  depend  upon  the  fact  that  every  current  is  sur- 
rounded by  a  magnetic  field  which  reacts  against  any  other  magnetic 
force  in  the  same  field.  This  is  illustrated  by  Fig.  I,  which  shows  the 
effect  of  a  wire  carrying  a  current  through  the  field  of  a  magnet.  The 
lines  of  force,  which  formerly  went  from  one  pole  to  the  other  by 


INSTRUMENTS.  219 

straight  lines  or  short  curved  ones,  as  shown  in  Fig.  2,  now  are 
stretched  by  the  effect  of  the  current,  and  the  wire  must  move  in  to- 
ward the  magnet  poles  in  order  to  relieve  the  tension.  In  the  figure 


FIG.  837-1.— EFFECT  OF  CURRENT  IN  MAGNET  FIELD. 


FIG.  837-2.— FIELD  BETWEEN  POLES  OF  MAGNET. 

the  current  is  shown  as  going  from  or  into  the  paper ;  if  the  direction 
of  the  current  had  been  opposite,  the  current  would  have  been  re- 
pelled, since  the  lines  of  force  would  then  have  been  crowded  together 
closer  on  the  inside  toward  the  magnet. 

838.  Does  the  whole  current  pass  through  the  coil  in  an  ammeter 
of  the  d'Arsonval  type? 

Only  in  those  for  very  small  currents.  In  order  to  be  sensitive, 
the  hair  springs  must  be  small  and  not  be  allowed  to  carry  much  cur- 
rent, or  they  may  get  hot  enough  to  lose  their  temper.  A  voltmeter 
takes  so  little  current  that  the  whole  of  it  can  safely  pass  through  the 
delicate  hair  springs  without  danger,  but  only  a  small  fraction  of  the 
current  through  an  ammeter  can  pass  through  the  springs. 


220 


ELECTRICAL   CATECHISM. 


839.  How  can  the  ammeter  be  made  to  measure  correctly  when 
only  part  of  the  current  goes  through  the  coil? 

The  coil  is  arranged  so  that  a  definite  proportion  of  the  whole  cur- 
rent passes  through  it.  A  large  conductor  of  low  resistance  is  con- 
nected directly  between  the  two  terminals  or  binding  posts  of  the  in- 
strument ;  the  coil  is  connected  as  a  shunt  around  a  definite  part  of 
this  main  conductor;  then,  since  the  two  are  connected  in  multiple 
and  each  branch  has  a  definite  resistance,  the  current  divides  between 
the  two  branches  directly  in  proportion  to  their  relative  conductivi- 
ties, or  inversely  according  to  their  resistances.  The  coil,  therefore, 
takes  a  definite  part  of  the  whole  current,  and  the  force  moving  it  and 
its  pointer  away  from  the  zero  position  is  directly  proportional  to  the 
whole  current. 

840.  Upon  what  basis  are  switchboard  ammeters  constructed, 
in  which  the  instrument  is  connected  by  small  wires  like  lampcord? 

In  these  instruments,  which  are  now  made  by  several  com- 
panies, a  "  shunt "  is  connected  directly  into  the  bus-bars, 
while  the  galvanometer  part  may  be  placed  wherever  desired. 

SHUNTED    AMMETER 


BUS- BAR 

FIG.  840.— SWITCHBOARD  AMMETER. 

The  part  usually  thought  of  as  the  instrument  is  really  a  d'Arson- 
val  galvanometer,  practically  the  same  thing  as  the  voltmeter,  and 
contains  a  permanent  magnet  with  a  coil  mounted  on  pivot  bear- 
ings and  controlled  by  hair  springs,  which  also  carry  the  current  into 
the  coil.  The  coil  with  its  hair  springs  and  the  connecting  cords  have 
a  definite  resistance;  when  these  are  carefully  connected  to  the 
"shunt"  in  the  bus-bars,  a  small,  but  definite  portion  of  the  whole 


INSTRUMENTS. 


221 


current  goes  through  the  cords  and  the  galvanometer,  which  thus 
measures  the  whole  current.  Some  instruments  have  several  shunts 
for  different  ranges. 

841.  What  advantage  is  there  in  this  kind  of  instrument? 

It  saves  considerable  expense  in  building  the  switchboard,  since 
the  bus-bars  do  not  have  to  be  carried  to  any  particular  place  to  suit 
the  instrument.  Also,  if  desired,  a  number  of  galvanometers  may 
be  connected  around  the  shunt,  so  that  the  current  may  be  read  from 
different  points.  It  is  also  possible  to  do  away  with  the  regular 
shunt  entirely  and  connect  the  galvanometer  cords  to  points  on  the 
bus-bars  themselves  (which  points  must  be  found  with  care),  and 
so  do  away  with  the  two  joints  in  the  bus-bars  otherwise  necessary. 
It  is  also  possible  to  use  the  same  galvanometer  for  measuring  the 
currents  in  several  different  circuits. 

842.  How  can  one  instrument  be  used  to  measure  currents  in 
different  circuits? 

By  having  a  double  pole  switch  with  several  sets  of  terminals,  like 
a  voltmeter  switch,  the  galvanometer  may  be  connected  around 
definite  points  on  several  circuits.  It  is  necessary  that  the  switch  be 


FIG.   842.— MULTIPLE  POINT  INSTRUMENT   SWITCH. 

made  with  great  care,  so  as  to  have  negligible  resistance  and  so  as  to 
make  sure  and  uniformly  good  contacts.  If  desired,  the  same  gal- 
vanometer may  be  arranged  so  as  to  have  a  variety  of  ranges. 

843.  How  can  the  range  of  current  measured  by  a  galvanometer 
be  varied? 

The  range  of  the  galvanometer  may  be  varied  by  simply  changing 
the  ratio  of  the  resistance  of  the  coil  with  its  connections,  as  com- 
pared with  that  of  the  "shunt."  A  given  deflection  or  motion  of  the 
coil  in  the  galvanometer  is  always  caused  by  the  same  current;  but 


222  ELECTRICAL   CATECHISM. 

that  current  may  be  any  desired  fraction  of  the  whole  current  in  the 
main  circuit,  since  the  whole  current  divides  itself  between  the 
"shunt"  and  the  galvanometer  circuit  in  inverse  proportion  to  their 
relative  resistances.  If  the  resistance  of  either  circuit  is  changed 
while  that  of  the  other  remains  constant,  the  ratio  of  the  currents 
changes  accordingly.  Usually  it  is  not  desirable  to  change  the  re- 
sistance of  the  galvanometer  circuit ;  the  only  alternative  is  to  change 
that  of  the  shunt.  For  a  simple  example,  suppose  that  when  the 
galvanometer  is  shunted  around  I  ft.  length  of  a  bus-bar  or  other 
conductor,  80  amps,  give  a  full  scale  deflection  of  the  galvanometer 
needle ;  if  the  same  galvanometer  is  shunted  around  10  ft.  of  the 
same  conductor,  the  relative  resistances  have  changed  so  that  the 
same  current,  as  before,  is  sent  through  the  galvanometer  when  the 
whole  current  is  a  trifle  more  than  8  amps.  In  the  same  way,  if  the 
galvanometer  is  shunted  around  100  ft.  of  the  conductor,  the  gal- 
vanometer will  give  a  full  scale  deflection  when  the  whole  current  is 
a  little  more  than  p.8  amp.  The  simplest  way  of  finding  the  exact 
length  of  conductor  to  take  as  a  shunt  for  a  given  range  of  current 
is  to  "cut  and  try,"  although  the  method  of  calculating  it  mathemati- 
cally is  not  very  difficult. 

844.  What  is  the  difference  between  an  ammeter  and  an  ampere- 
meter? 

There  is  no  difference ;  some  prefer  one  word  and  some  prefer  the 
other.  Ampere-meter  is  more  correct  from  an  etymological  stand- 
point, but  most  people  seem  to  prefer  the  abbreviated  form. 

845.  What  is  the  difference  between  an  ammeter  and  a  current 
indicator? 

A  current  indicator  is  usually  a  cheap  form  of  ammeter  whose 
readings  are  not  claimed  to  be  very  accurate.  For  most  plants  it  is 
not  necessary  to  know  the  current  exactly  and  a  cheap  ammeter  is 
good  enough.  The  voltage  is  the  factor  of  importance,  and  it  is  bet- 
ter to  put  more  money  into  a  good  voltmeter  and  less  into  the  am- 
meter. 

846.  Is  there  any  difference  between  a  voltmeter  and  a  volta- 
meter? 

Yes;  there  is  a  wide  difference.  A  voltmeter  is  an  instrument 
similar  to  an  ammeter,  except  that  it  is  arranged  to  measure  voltage 
or  pressure.  The  voltameter,  on  the  other  hand,  is  an  instrument 
based  upon  the  chemical  effects  of  the  current  and  measures  the 
product  of  current  by  time.  The  Edison  chemical  meter  is  an  ex- 


INSTRUMENTS.  223 

ample  of  a  voltameter  used  to  determine  the  ampere-hours  of  current 
used  by  a  consumer.     (See  Nos.  815  to  821.) 

847.  What  is  the  arrangement  of  voltmeters  that  have  different 
scales  corresponding  to  different  binding  posts? 

Such  instruments  usually  have  only  one  galvanometer  coil,  but 
have  a  number  of  different  resistances  in  series.  One  terminal  con- 
nects directly  with  the  galvanometer  coil ;  the  terminal  for  the  lowest 
scale  is  connected  directly  to  the  other  terminal  of  the  coil,  or  often 
to  a  small  adjusted  resistance  coil  in  series  with  the  coil;  the  next 
higher  terminal  connects  with  the  circuit  after  it  has  passed  through 


FIG.  847.— DOUBLE-SCALE  VOLTMETER. 

a  second  and  higher  resistance.  The  theory  of  the  instrument  is  as 
indicated  in  Nos.  848  and  931.  A  certain  current  is  required  to  move 
the  pointer  to  a  certain  mark ;  but  the  pressure  or  voltage  necessary 
to  send  such  current  through  the  instrument  depends  upon  the  re- 
sistance of  the  circuit ;  hence  each  terminal  governing  the  resistance 
in  series  with  the  galvanometer  coil  determines  which  scale  shall  be 
read  for  the  true  voltage.  For  "  multipliers  "  see  No.  931. 

848.     What  is  the  difference  between  a  voltmeter  and  an  ammeter? 

An  ammeter  measures  current,  while  a  voltmeter  measures  pres- 
sure (or  E.M.F.).  As  actually  constructed,  most  voltmeters  are 
simply  special  forms  of  ammeters.  From  Ohm's  law,  the  current 
through  a  given  circuit  equals  the  pressure  at  its  terminals  divided  by 
its  resistance ;  if  a  high  resistance  is  connected  in  series  with  a  sen- 
sitive ammeter  that  will  measure  very  small  currents,  then  the  cur- 
rent passing  through  the  circuit  is  directly  proportional  to  the  pres- 
sure or  voltage  at  its  terminals,  and  the  instrument  may  be  calibrated 
to  read  volts. 


224  ELECTRICAL  CATECHISM. 

849.  What  is  meant  by  calibrating  an  instrument? 

Calibration  refers  to  the  construction  of  the  scale  of  the  instru- 
ment, or  to  the  determination  of  the  value  of  different  positions  on 
the  scale.  For  example,  suppose  that  an  instrument  has  a  resistance 
of  10,000  ohms  and  that  o.ooi  amp.  (or  i  milliampere)  causes  the 
pointer  to  move  to  a  point,  say,  i  in.  from  the  zero  or  starting  point. 
The  pressure  necessary  to  send  o.ooi  amp.  through  10,000  ohms  is 
10  volts,  since  E  =  C  X  R-  That  point  on  the  scale  might  be  marked 
either  as  o.ooi  amp.  or  as  10  volts.  In  a  similar  way,  the  value  of 
every  other  point  on  the  scale  might  be  determined.  If  the  current 
values  are  marked  on  the  scale,  we  say  that  the  instrument  is  cali- 
brated to  read  amperes  or  milliamperes ;  while  if  the  scale  were 
marked  to  show  the  pressure  at  the  terminals  of  the  instrument 
necessary  to  give  the  same  readings,  we  would  say  that  it  was  cali- 
brated to  read  volts.  (See  No.  924.) 

850.  Why  does  a  voltmeter  have  a  high  resistance? 

For  the  sake  both  of  economy  and  of  accuracy.  By  making  the 
galvanometer  very  sensitive,  less  current  is  required  to  operate  it, 
and,  as  a  result,  more  resistance  is  needed  to  keep  the  current  down 
to  that  required.  Some  voltmeters  take  so  much  current  that  they 
soon  become  heated  to  a  dangerous  degree  if  left  in -circuit  continu- 
ously, and  therefore  the  switch  or  push-button  should  be  left  open 
except  when  the  instrument  is  in  actual  use.  Even  if  the  instrument 
does  not  become  so  hot  as  to  endanger  its  insulation,  its  resistance 
changes  and  it  becomes  warm;  this  allows  less  and  less  current  to 
pass  through  the  instrument,  and  it  then  indicates  a  lower  voltage 
than  that  actually  at  its  terminals. 

851.  Why  does  a  voltmeter  with  low  resistance  get  a  poor  repu- 
tation even  if  the  switch  is  closed  only  when  in  actual  use? 

A  voltmeter  with  low  resistance  takes  considerable  current.  If 
the  circuit  whose  voltage  is  being  measured  has  comparatively  high 
resistance,  the  additional  current  taken  by  the  voltmeter  may  cause 
an  appreciably  greater  drop  on  the  line,  so  that  the  voltage  at  the 
point  tested  may  be  considerably  lower  when  the  voltmeter  is  i. 
circuit. 

852.  Why  are  the  division  marks  omitted  in  the  lower  parts  of 
the  scales  of  many  instruments? 

Some  instruments  are  intended  for  use  with  currents  or  pressures 
that  vary  only  through  narrow  limits,  and  the  scale  is  marked  only 
on  those  parts  to  save  expense.  Other  instruments  are  not  accurate 


INSTRUMENTS. 


225 


for  small  readings,  and  so  the  scales  are  not  calibrated  below  a  point 
where  the  instrument  becomes  unreliable. 

853.  Why  are  such  instruments  not  reliable  for  small  readings? 
Partly  because  the  friction  of  the  moving  parts  is  relatively  great, 

and  partly  because  many  instruments  depend  upon  the  attraction 
or  repulsion  of  soft  iron  cores,  which  is  not  constant  for  small  cur- 
rents at  different  times  on  account  of  hysteresis.  (See  No.  763.) 

854.  What  instruments  are  based  upon  the  attraction  of  a  core 
into  a  coil? 

Familiar  examples  are  the  ammeters  used  with  Brush  arc  dynamos 
and  those  formerly  sold  with  Edison  dynamos  and  with  Westing- 
house  alternators.  One  of  the  latter  is  illustrated  in  the  figure,  in 


FIG.  854.— SOLENOID  AMMETER. 

which  the  bundle  of  fine  iron  wires  is  seen  hung  from  one  end  of  a 
sort  of  balance  having  a  counterweight  at  the  further  end  and  a  long 
pointer  hung  below. 


226  ELECTRICAL  CATECHISM. 

855.  What  is  the  principle  of  the  eccentric  instruments  of  Thom- 
son-Houston and  Fort  Wayne  companies? 

These  instruments,  illustrated  in  the  figure,  have  a  stirrup  of  soft 
iron  hung  eccentrically  in  a  coil  in  such  a  way  that  the  iron  stirrup 
is  furthest  from  the  coil  when  no  current  is  passing;  when  current 
passes  through  the  coil,  the  soft  iron  strip  becomes  magnetized;  the 


FIG.  855.— ECCENTRIC  AND   INCLINED   COIL   INSTRUMENTS. 

magnetic  lines  of  force  about  the  coil  always  tend  to  take  the  easiest 
path ;  in  this  case,  the  easiest  path  is  through  the  iron  strip,  and  the 
path  becomes  shorter  and  easier  when  the  strip  is  closer  to  the  coil. 
The  attraction  between  the  coil  and  iron  causes  the  iron  to  move  so 
as  to  approach  the  coil,  but  it  can  only  approach  the  coil  by  turning 
around  on  the  pivots.  The  weight  is  so  adjusted  as  to  tend  to  keep 
the  strip  furthest  from  the  coil  and  thus  gravity  opposes  the  at- 
traction of  the  strip.  These  are  now  obsolete. 

The  "  inclined  coil  "  instruments  of  the  General  Electric  Company 
are  based  on  a  similar  principle,  the  plane  of  a  soft  iron  piece  being 
inclined  from  the  axis  of  the  coil  and  coming  toward  such  axis  as  it 
rotates  against  a  spring. 

856.     What  is  the  principle  of  the  "magnetic  vane"  instruments? 

The  magnetic  vane  instruments  have  two  soft  iron  cores,  one  of 
which  is  fixed  and  repels  the  other  one,  which  is  pivoted  and  free 
to  move.  The  two  iron  pieces  repel  each  other,  since  they  are  mag- 
netized in  the  same  direction ;  also,  since  the  repelling  force  is  great- 
est when  the  two  are  closest  together,  the  force  moving  the  needle  or 
pointer  is  considerable  even  with  small  currents,  and  the  instrument 
may  be  calibrated  from  a  point  near  the  zero  of  the  scale. 


CHAPTER  VIII. 


ELECTRICAL  MEASUREMENTS. 

(Direct  Current* ) 

900.  How  is  current  measured ? 

Current  is  measured  by  placing  an  ammeter  or  its  equivalent 
directly  in  the  circuit  so  that  the  current  to  be  measured  shall  pass 
through  the  instrument. 

901.  How  is  electromotive  force  measured? 

Electromotive  force  or  voltage  is  measured  by  connecting  a  volt- 
meter or  its  equivalent  to  the  two  points  whose  difference  of  poten- 
tial is  desired.  The  common  method  of  using  the  instruments  is 


Am. 
Lamps 

HDynJ 

Vm. 

D 

D 

D 

D 

D 

D 

0 

o 

V 

FIG.  901.— MEASURING  CURRENT  AND  VOLTAGE. 

shown  in  the  figure,  in  which  current  from  the  dynamo  passes 
through  the  ammeter  and  then  through  the  lamps,  the  pressure  or 
voltage  being  measured  by  the  voltmeter  connected  across  the  line. 

902.  How  can  one  tell  whether  an  instrument  is  an  ammeter  or 
a  voltmeter? 

Almost  all  instruments  are  marked.  If  the  scale  is  marked  to  read 
amperes,  or  if  the  name  on  the  plate  states  that  the  instrument  is  an 
ammeter,  an  ampere-meter  or  a  current  indicator,  one  may  be  sure 
it  is  intended  for  measuring  current  and  not  volts.  So,  if  the  scale 
is  marked  to  read  volts,  or  if  the  name  plate  states  that  it  is  a  volt- 
meter, pressure  indicator  or  potential  indicator,  it  is  for  measuring 
voltage  and  not  current.  If  the  instrument  has  no  marks  of  identifi- 
cation, the  size  of  the  terminals  or  binding  posts  will  be  an  indica- 
tion, ammeters  usually  having  comparatively  large  terminals,  while 
voltmeters  have  small  ones.  One  can  generally  get  some  idea  of  the 


228  ELECTRICAL   CATECHISM. 

size  of  the  conductors  in  the  coil  either  by  looking  through  the  glass 
front  or  by  taking  off  the  cover.  If  the  wire  is  about  the  size  of  a 
coarse  thread  or  smaller,  the  instrument  is  a  voltmeter  or  a  milL- 
ampere-meter.  If  the  wire  is  larger,  the  instrument  is  an  ammeter. 
If  the  instrument  has  two  large  and  two  small  terminals,  it  is  a  watt- 
meter, or  possibly  a  combined  ammeter  and  voltmeter. 

903.  What  would  result  if  one  attempted  to  use  an  ammeter  for  a 
voltmeter? 

The  ammeter  has  so  small  resistance  that  if  it  were  connected  be- 
tween two  points  having  a  considerable  difference  of  potential,  a 
large  current  would  flow  in  accordance  with  Ohm's  law,  and  would 
be  apt  to  damage  not  only  the  instrument,  but  other  parts  of  the  cir- 
cuit as  well. 

904.  What  would  result  if  one  attempted  to  use  a  voltmeter  as 
an  ammeter ? 

The  voltmeter  has  so  high  resistance  that  it  would  not  allow  any 
appreciable  current  to  pass  unless  the  E.M.F.  were  very  high,  in 
which  case  the  voltmeter  would  be  burned  out.  Sometimes  the  volt- 
meter may  be  used  as  a  delicate  ammeter  for  measuring  quite  small 
currents. 

905.  Is  an  instrument  liable  to  injury  if  not  connected  properly 
into  the  circuit? 

Usually  instruments  are  not  injured  if  the  polarity  is  reversed  so 
that  the  current  flows  in  the  wrong  direction.  If  the  instrument  is 
intended  for  alternating  currents,  of  course  its  readings  are  inde- 
pendent of  the  direction  of  the  current.  If  the  instrument  depends 
upon  the  action  of  a  permanent  magnet,  the  pointer  simply  moves 
in  the  wrong  direction  and  goes  against  a  stop. 

906.  How  can  one  be  sure  that  the  current  will  not  be  too  much 
for  the  instrument? 

In  the  first  place  he  must  exercise  some  judgment,  as  a  knowledge 
of  what  load  is  on  the  circuit  will  ordinarily  enable  one  to  estimate 
the  probable  current  or  voltage  to  be  expected.  He  must  be  careful 
that  he  does  not  try  to  use  an  ammeter  for  a  voltmeter  or  a  voltmeter 
for  an  ammeter,  unless  he  knows  exactly  what  he  is  doing.  If  the 
voltmeter  has  two  scales,  he  should  first  try  the  circuit  with  the  in- 
strument connected  for  use  with  the  higher  scale  to  learn  if  he  can 
safely  use  the  lower  scale. 


MEASUREMENTS.  229 

907.  When  a  voltmeter  has  two  scales,  how  can  one  'tell  which 
terminals  to  use  to  get  the  right  scale? 

Voltmeters  with  two  scales  usually  have  three  terminals,  one  on 
one  side  of  the  instrument  and  two  on  the  other  side.  The  single 
terminal  is  used  for  both  scales.  One  of  the  two  terminals  on  the 
other  side  is  usually  smaller  than  the  other,  and  one  can  easily  re- 
member that  the  smaller  terminal  goes  with  the  smaller  scale. 

908.  What  would  be  the  effect  of  sending  too  much  current 
through  an  instrument? 

The  pointer  would  be  thrown  violently  against  the  upper  stop, 
with  so  much  force  that  it  would  probably  become  permanently  bent. 
If  the  excessive  current  is  allowed  to  continue  through  the  instru- 
ment for  any  length  of  time,  it  is  likely  to  heat  the  wire  to  a  danger- 
out  temperature  and  destroy  the  insulation,  or,  perhaps,  melt  the 
wires.  The  same  effects  are  also  produced  if  a  voltmeter  is  con- 
nected across  a  circuit  of  too  high  voltage,  since  that  would  send  too 
much  current  through  the  coils.  In  the  case  of  a  static  voltmeter  in 
which  no  current  flows,  too  high  voltage  is  liable  to  jump  across  and 
short-circuit  the  instrument  and  the  line. 

909.  Is  it  better  to  close  the  circuit  at  an  ammeter  or  at  some 
other  point  in  the  circuit? 

The  circuit  should  not  be  made  or  broken  at  the  ammeter  ter- 
minals, since  it  is  not  easy  to  close  the  circuit  without  making  an  un- 
certain contact,  which  causes  arcing  and  burns  the  terminal.  Also, 
it  is  not  possible  to  open  a  circuit  at  the  ammeter  without  drawing  an 
arc,  which  burns  and  disfigures  the  terminal.  It  is  a  good  plan  to  have 
the  ammeter  near  a  switch,  that  one  may  see  what  current  flows  at 
the  instant  the  circuit  is  closed,  so  that  if  anything  is  wrong  he  can 
open  the  circuit  before  considerable  damage  is  done. 

910.  Is  the  reliability  of  an  instrument  affected  if  the  terminals 
are  burned  and  blistered? 

The  instrument  itself  might  not  be  injured  at  the  time,  but  a  dis- 
figured instrument  is  almost  sure  to  be  looked  upon  as  being  an  old 
one  and  unreliable,  hence  it  is  more  liable  to  abuse.  The  man  using 
a  disfigured  instrument  is  also  sure  to  think  that,  as  the  instrument 
is  a  poor  one,  it  makes  little  difference  whether  he  does  accurate 
work,  since  he  can  not  depend  upon  the  instrument,  anyhow.  Thus 
the  practice  of  using  the  ammeter  as  a  switch  for  closing  and  opening 
the  circuit  is  very  apt  to  lead  to  poor  work  from  those  thus  using  it. 


230  ELECTRICAL   CATECHISM. 

911.  In  what  way  is  an  instrument  liable  to  be  abused? 

By  careless  use  as  a  switch,  by  rough  handling,  and  by  its  use  with 
excessive  curents  or  voltages. 

912.  How  does  rough  handling  injure  an  instrument? 

The  best  voltmeters  and  ammeters  are  made  much  like  a  watch, 
with  small  pivots  which  rotate  on  jewel  bearings.  Rough  handling 
is  apt  to  crack  the  jewels  or  to  dull  the  ends  of  the  pivots,  or  to  spring 
the  parts.  Rough  handling  is  also  liable  to  weaken  the  permanent 
magnet,  if  the  instrument  has  one,  which  will  make  the  instrument 
read  too  low  or  too  high. 

913.  What  kind  of  instruments  read  too  high  if  the  permanent 
magnet  is  weakened? 

Instruments  such  as  the  Deprez  galvanometer  or  Blondel  oscillo- 
graph (Fig.  1505),  in  which  the  pointer  or  mirror  is  attached  to  a 
piece  of  soft  iron  that  is  held  toward  the  zero  point  by  a  stationary 
permanent  magnet  and  is  deflected  by  a  coil,  whose  magnetizing 
effect  is  at  right  angles  to  that  of  the  magnet.  When  the  magnet 
becomes  weak,  the  relative  pull  of  a  given  current  through  the  coil 
becomes  greater,  and  the  pointer  is  then  deflected  too  far,  thus  mak- 
ing the  instrument  read  too  high. 

914.  What  kind  of  instruments  read  too  low  if  the  permanent 
magnet  is  weakened? 

Those  like  the  Weston,  Whitney  or  American,  in  which  the  coil 
rotates  in  the  field  of  the  permanent  magnet.  In  such  instruments 
the  pull  of  the  coil  is  opposed  by  a  pair  of  springs.  The  pull  of  the 
coil  is  proportional  to  the  product  of  the  strength  of  the  magnet 
and  the  ampere-turns  in  the  coil.  If  then  the  magnet  becomes 
weaker,  the  pull  between  the  coil  and  the  magnet  becomes  weaker 
for  a  given  current,  and  since  the  strength  of  the  opposing  springs 
has  not  been  affected,  the  instrument  reads  too  low. 

915.  Is  an  instrument  liable  to  give  incorrect  readings  even  if  it 
is  in  good  order? 

Yes,  to  some  extent.  Most  instruments  are  more  or  less  affected 
by  outside  magnetic  influences.  Many  instruments  give  correct 
readings  only  when  approximately  level  or  when  vertical,  according 
to  the  style  of  instrument. 

916.  How  does  leveling  affect  instruments? 

Some  instruments,  such  as  the  Siemens  electrodynamometer  (see 
No.  825)  must  be  carefully  leveled  so  that  the  moving  coil  shall  hang 
exactly  in  the  same  position  with  reference  to  the  stationary  coil,  for 


MEASUREMENTS.  231 

the  magnetic  field  of  each  coil  varies  in  different  positions,  and  it  is 
necessary  for  the  two  to  occupy  exactly  the  same  relative  positions 
as  when  the  instrument  was  calibrated.  In  many  instruments  of  a 
more  portable  form  the  moving  parts  are  balanced  for  only  one 
position,  either  for  horizontal  or  for  vertical  use.  If  the  instrument 
is  placed  in  any  other  position,  the  pointer  is  liable  to  stop  at  some 
other  place  than  at  the  zero  point,  and  being  wrong  at  the  starting 
point,  every  other  indication  of  the  pointer  is  liable  to  be  incorrect. 

917.  How  are  instruments  affected  by  outside  magnetic  influ- 
ences. 

All  instruments  depending  upon  the  magnetic  effect  of  the  current 
are  more  or  less  sensitive  to  outside  magnetic  fields,  for  these  either 
weaken  or  strengthen  the  magnetic  field  inside  the  instrument.  Gen- 
erally speaking,  the  weaker  the  regular  magnetic  field  the  more  easily 
it  is  affected  by  outside  disturbances.  Delicate  galvanometers,  which 
depend  upon  the  earth's  magnetic  field,  are  sometimes  sensitive  to 
the  running  of  a  dynamo  hundreds  of  feet  distant  or  to  the  return 
currents  from  a  trolley  car  some  thousand  feet  distant.  Almost  every 
instrument  is  affected  if  close  to  a  dynamo  or  a  motor.  Instruments 
having  permanent  magnets  are  liable  to  affect  one  another  unless 
kept  at  least  2  ft.  apart.  The  magnetic  field  around  conductors  carry- 
ing heavy  currents  is  strong  enough  to  affect  instruments  of  the  elec- 
trodynamometer  type,  and  sometimes  even  those  of  the  d'Arsonval 
(see  No.  834)  type.  In  no  case  should  instruments  be  placed  within 
2  or  3  ft.  of  a  dynamo  or  a  motor. 

918.  How  can  one  tell  whether  an  instrument  is  being  affected 
by  a  stray  magnetic  field? 

By  turning  the  instrument  around,  the  effect  of  the  outside  dis- 
turbing field  will  be  reversed,  so  that  if  the  reading  was  too  high 
before,  it  will  then  be  too  low.  The  average  of  the  two  readings 
taken  with  the  instrument  in  two  positions  will  be  close  to  the  correct 
reading.  If  the  instrument  does  not  depend  upon  the  field  of  a  per- 
manent magnet,  its  readings  may  be  corrected  by  simply  reversing 
the  current  through  the  instrument,  instead  of  reversing  the  instru- 
ment itself. 

919.  Are  instruments  measuring  alternating  currents  affected  by 
stray  fields? 

That  depends  upon  the  nature  of  the  disturbance.  If  the  outside 
field  is  uniform  in  strength  and  direction,  it  will  not  ordinarily  affect 
the  readings  of  an  instrument  measuring  alternating  currents.  But 
if  the  outside  field  is  due  to,  or  is  affected  by,  the  alternating  current, 


232  ELECTRICAL   CATECHISM. 

it  may  affect  the  instrument.  An  instrument  for  measuring  alternating 
currents  is  liable  to  be  affected  by  the  field  due  to  the  current  in  the 
wires  leading  to  and  from  the  instrument.  Such  instruments  are 
liable  to  be  affected  by  eddy  current^  induced  in  any  solid  masses  of 
metal  that  may  be  near  the  coils.  Any  pieces  of  iron  near  the  instru- 
ments are  liable  to  become  more  or  less  magnetized  by  the  current, 
which  will  result  in  strengthening  the  magnetic  field  in  the  instru- 
ment itself. 

920.  Do  instruments  calibrated  for  use  with  alternating  currents 
give  correct  readings  when  used  with  direct  currents? 

Instruments  of  the  electrodynamometer  type  give  equally  correct 
readings  with  direct  or  alternating  currents,  except  that  they  are 
more  liable  to  be  affected  by  outside  magnetic  disturbances  when 
used  with  direct  currents.  Instruments  having  iron  cores  or  needles 
give  higher  readings  with  direct  than  with  alternating  currents ;  with 
alternating  currents,  the  higher  the  number  of  alternations  the  lower 
the  reading.  Electrodynamometers  are  independent  of  the  fre- 
quency. The  type  shown  in  the  illustration  with  No.  825,  has  been 
improved  upon  in  modern  instruments. 

921.  How  can  one  measure  the  current  without  opening  the  cir- 
cuit? 

If  there  is  a  switch  in  the  circuit  in  a  convenient  place,  an  ammeter 
may  be  connected  around  the  switch;  when  the  switch  is  opened, 
the  whole  current  will  pass  through  the  ammeter.  This  method  is 
frequently  used  in  measuring  the  current  in  an  arc-light  circuit 
either  at  the  station  or  out  on  the  line.  At  the  station  the  ammeter 
13  shunted  around  one  of  the  switchboard  cables,  which  is  then  pulled 
out.  Out  on  the  line  one  may  connect  an  ammeter  to  two  sides  of 
a  coupling  and  then  loosen  the  joint.  Of  course,  great  care  must  be 
taken  not  to  open  the  circuit,  and  also  not  to  make  connection  with 
the  ground,  for  there  is  great  danger  of  getting  killed  or  of  putting 
out  all  the  lights  on  the  circuit. 

922.  How  can  the  current  be  measured  when  there  is  no  place 
for  cutting  in  an  ammeter? 

Either  of  two  methods  may  be  employed,  each  a  "fall  of  potential" 
method.  If  one  can  secure  a  piece  of  conductor  the  same  as  that 
through  which  the  current  is  passing,  he  may  use  a  low-reading 
voltmeter  (a  millivoltmeter)  and  measure  the  difference  of  potential 
between  two  points  of  the  conductor  some  definite  distance  apart, 
say  2  or  3  ft.  The  millivoltmeter  is  then  connected  between  two 
points,  an  equal  distance  apart,  on  another  conductor  the  same  size 


MEASUREMENTS.  233 

and  material  as  the  first ;  now  send  a  measured  current  through  the 
second  conductor  and  adjust  the  current  until  the  millivoltmeter 
shows  the  same  deflection  as  before.  Then,  since  the  resistances  of 
the  two  pieces  of  conductor  are  the  same,  and  the  fall  of  potential 
is  the  same  in  each,  it  follows  that  the  current  is  the  same.  Instead 
of  sending  an  equal  current  in  the  two  cases,  a  smaller  current  may 
be  taken  in  the  second  case,  and  the  currents  will  then  be  propor- 
tional to  the  two  readings  on  the  millivoltmeter. 

923.  How  can  the  current  be  measured  when  a  second  piece  of 
conductor  is  not  accessible,  as  in  the  case  of  measuring  the  current 
in  a  rail? 

In  such  a  case  another  modification  of  the  fall  of  potential  method 
may  be  used.  Secure  a  storage  cell,  S,  capable  of  delivering  a  cur- 
rent as  large  as  that  to  be  measured  and  have  an  adjustable  re- 
sistance, r,  for  controlling  its  current.  Place  an  ammeter,  A,  in  the 
circuit  of  the  cell  and  then  fasten  the  terminals  of  the  cell  circuit  to 


r 

Am.EIec. 
FIG.  923.-MEASURING  CURRENT  BY  FALL  OF  POTENTIAL, 

the  rail,  CD,  or  other  conductor.  Attach  the  terminals  of  a  milli- 
voltmeter or  more  sensitive  galvanometer  to  two  points,  EF ,  on  the 
rail  inside  of  the  points  where  the  ^.ell  is  attached,  and  adjust  the 
current  from  the  cell  until  the  galvanometer  gives  no  deflection. 
The  fall  of  potential  due  to  the  original  current  in  the  rail  is  now 
exactly  balanced  by  that  due  to  the  current  from  the  cell,  and  the 
latter  current  may  be  measured  by  the  ammeter.  The  complete 
theory  of  this  method  is  somewhat  complicated  and  the  practice  of 
it  involves  experimental  difficulties. 

924.     What  is  meant  by  calibrating  an  instrument ? 

The  calibration  of  an  instrument  is  the  process  of  determining  the 
value  of  the  current  or  voltage  required  to  move  the  indicator  to  any 
or  all  parts  of  the  scale.  This  may  be  done  either  in  making  a  new 
scale  or  in  checking  an  instrument  which  has  been  in  use.  The 
calibration  may  be  simply  a  comparison  with  a  standard  instrument, 
or  it  may  require  the  use  or  construction  of  absolute  standards.  (See 
No.  849.) 


234 


ELECTRICAL  CATECHISM. 


925.  How  are  absolute  standards  used? 

Absolute  standards  of  current  or  E.M.F.  depend  upon  the  skilful 
use  of  chemical  solutions  of  absolute  purity  and  handled  in  accord- 
ance with  certain  specifications  which  have  been  adopted  by  govern- 
ment. (See  Nos.  227  and  228).  It  is  so  much  trouble  and  expense 
to  use  the  absolute  standards  that  it  is  more  common  to  use  first-class 
instruments,  which  were  originally  calibrated  by  reference  to  abso- 
lute standards,  and  whose  calibration  is  checked  at  intervals.  Any 
one  contemplating  such  work  should  consult  Gray's  book  on  Ab- 
solute Measurements  and  Transactions  of  American  Institute  of 
Electrical  Engineers,  vol.  10,  pp.  13  to  26. 

926.  How  is  an  ammeter  calibrated  by  reference  to  a  standard 
ammeter? 

The  two  instruments  are  connected  in  series,  and  the  same  current 
is  sent  through  both.  The  current  is  measured  on  the  standard  in- 
strument and  the  deflection  of  the  pointer  of  the  other  is  noted.  The 
strength  of  the  current  is  then  changed  and  readings  are  taken  on 
both  instruments  again.  In  this  way,  as  many  points  as  are  desired 
are  established  and  a  scale  is  made.  For  such  work  it  ^is  desirable 
to  use  a  storage  battery  so  that  the  current  may  remain  constant  dur- 


Standard 
Ammeter 


Test 
Ammeter 


V 


Lamps 


FIG.  926.— CALIBRATING  AN  AMMETER. 

ing  each  set  of  readings.  The  'two  instruments  should  be  placed  far 
enough  apart  so  that  the  magnetic  field  from  one  does  not  affect  the 
other  (see  Nos.  752  to  755),  and  each  should  be  carefully  leveled. 
It  is  well  to  compare  the  two  instruments  with  increasing  currents 
and  then  with  decreasing  currents  to  check  against  errors  and  to 
see  how  far  the  instrument  is  affected  by  friction  or  by  hysteresis 
(see  No.  760).  Instead  of  using  a  standard  ammeter,  a  standard 
resistance  and  millivoltmeter  are  sometimes  used  for  measuring  the 
current  for  calibration.  Low  voltage  is  economical  for  such  work. 

927.  How  can  current  be  measured  by  a  resistance  and  milli- 
voltmeter? 

This  is  essentially  the  same  as  the  use  of  a  galvanometer  and  shunt 
(see  No.  839).  By  Ohm's  law  (see  Nos.  315  to  326)  the  fall  of 


MEASUREMENTS. 


235 


potential  or  drop  through  a  conductor  equals  the  product  of  its  re- 
sistance by  the  current  passing.  If  then  the  resistance  is  constant  and 
is  known,  the  current  may  be  calculated  by  measuring  the  voltage 


FIG.    927.— MEASURING   CURRENT    BY   FALL   OF    POTENTIAL. 

between  its  terminals.  Standard  resistances  may  be  obtained  with 
great  accuracy  and  capable  of  carrying  considerable  currents  without 
appreciable  change  of  resistance. 

928.  Hozv  are  voltmeters  calibrated? 

The  simplest  method  is  by  comparing  with  a  standard  voltmeter, 
the  two  instruments  being  connected  in  multiple  so  that  both  are  sub- 
jected to  the  same  voltage,  as  suggested  in  Fig.  930.  The  voltage 
between  the  terminals  is  then  changed  to  different  values  and  read- 
ings on  the  two  instruments  are  compared. 

929.  How  are  the  results  of  a  calibration  used? 

A  new  scale  for  the  instrument  may  be  made.  A  less  difficult 
process  is  to  construct  a  curve  showing  the  corrections  to  be  made 
when  using  the  instrument.  The  results  of  the  test  may  be  left  in 
the  form  of  a  table ;  but  as  it  is  necessary  to  find  values  other  than 
those  in  the  test,  it  is  usually  easier  to  get  these  from  a  curve  than 
by  interpolating  from  the  table.  Suppose  that  the  accompanying 
table  gives  the  results  of  a  test.  If  in  later  use,  the  instrument  should 
read  80  volts,  the  table  shows  that  the  correct  pressure  is  78.6  volts ; 
if  it  reads  82  volts,  we  could  add  to  78.6  two-fifths  of  the  difference 
between  78.6  and  83,  which  woujd  make  80.36  volts.  This  is  likely  to 

CALIBRATION  OF  VOLTMETER  No.  929 


No  929 
Readings 

Standard  Volts 

Volts  Error 

No.  929 
Readings 

• 
S  andard  Volts 

Volts  Error 

IO 

10.2 

+    .2 

70 

69.0 

—  1.0 

20 

20.9 

+    .9 

80 

78.6 

—1.4 

3<> 

31.3 

+  1-3 

90 

87.8 

—  2.2 

40 

40.8 

+   .8 

IOO 

97-5 

—2-5 

50 

50.1 

+   .1 

no 

107.6 

—2.4 

60 

59-7 

~~-3 

120 

117.8 

2.2 

236 


ELECTRICAL   CATECHISM. 


require  a  calculation  each  time,  so  that  it  is  easier  to  construct  a 
curve  from  which  intermediate  values  may  be  taken  by  inspection. 
One  ir^thod  is  to  compute  a  column  of  differences  betwen  the  read- 
ings of  tne  two  instruments,  and  then  plot  a  curve  in  which  horizon- 


42. 


Am.Elec. 
FIG.   929A.— CALIBRATION    CURVE. 


tal  distances  represent  scale  readings,  and  vertical  distances  show 
the  amount  to  be  added  or  subtracted  to  give  the  true  value.  An- 
other plan  is  to  draw  a  curve  in  which  horizontal  distances  represent 
the  indications  of  the  instrument  tested,  while  vertical  distances  rep- 


130 
120 
110 
100 
90 
80 
«  70 

2« 

50 
40 
30 
20 
10 


10      20      30      40      50      60      70      80     90     100     110    120    130    140     150 
Readings 

FIG.  929s.— CALIBRATION  CURVE. 

resent  the  correct  values.  The  departure  of  this  curve  from  a  line 
drawn  at  45  degs.  shows  the  amount  of  correction  in  any  part  of  the 
scale.  The  accompanying  table  and  curves  illustrate  a  case. 


MEASUREMENTS, 


23? 


930.  How  can  different  voltages  be  obtained  for  calibrating  volt- 
meters? 

When  a  battery  is  available,  a  convenient  method  is  to  connect  one 
common  terminal  of  the  two  instruments  to  one  end  of  the  battery 
and  moving  the  other  common  terminal  from  cell  to  cell,  so  as  to 
secure  as  much  pressure  as  desired  between  the  terminals  of  the  in- 
struments. A  more  common  method  is  to  connect  a  suitable  re- 
sistance across  a  constant  potential  circuit;  for  example,  15  ft.  or  20 
ft.  of  No.  30,  or  about  100  ft.  of  No.  24  German  silver  wire  may 
be  connected  across  a  circuit  with  about  100  volts.  A  small  current 
will  flow  through  the  wire,  and  as  its  resistance  per  foot  is  uniform, 


Resistance 


FIG,  930.— FALL  OF  POTENTIAL  IN  A  WIRE. 

the  fall  of  potential  will  correspond.  By  shunting  the  voltmeters 
around  more  -or  less  of  this  wire,  uiy  pressure  desired  may  be  ob- 
tained. It  is  important  that  the  switches  or  push-buttons  of  both  in- 
struments be  closed  when  readings  are  taken,  for  the  voltage  is  apt 
to  change  if  only  one  is  closed  at  a  time. 

931.  How  can  a  voltmeter  be  used  to  measure  higher  voltage 
than  its  ordinary  range? 

The  range  of  a  voltmeter  is  doubled  by  placing  it  in  series  with 
an  equal  resistance.  For  example,  if  a  voltmeter  reading  to  150  volts 
has  a  resistance  of  20,000  ohms,  it  will  read  to  300  volts  when  in 
series  with  an  added  resistance  of  20,000  ohms.  This  is  true  only 
for  instruments  that  take  current,  and  does  not  hold  true  for  electro- 
static instruments.  The  reason  is  that  an  ordinary  voltmeter  is 
really  a  sort  of  ammeter  with  high  resistance  (see  Nos.  826  and  848), 
and  if  the  resistance  of  the  circuit  is  doubled,  twice  as  much  pressure 
is  required  to  send  the  same  current  through.  By  having  several 
resistances,  the  voltmeter  may  be  given  any  range  desired.  Such 
resistances  are  called  "  multipliers  "  (see  Nos.  847  and  932).  With 
alternating  currents,  transformers  are  used  (see  Nos.  1405,  1407). 


238  ELECTRICAL   CATECHISM. 

932.  What  is  the  fractional  method  of  'measuring  pressures 
higher  than  the  range  of  a  voltmeter? 

Connect  in  series  a  number  of  resistances,  such  as  incandescent 
lamps,  and  couple  the  ends  of  the  circuit  to  the  line  whose  voltage  is 
to  be  measured.  There  should  be  enough  of  the  lamps  in  the  string  so 
that  not  too  much  current  will  pass  and  destroy  the  lamps,  and,  if  pos- 
sible, the  lamps  should  be  of  the  same  candle-power  voltage  and  effi- 
ciency, so  as  to  have  equal  resistance.  A  small  current  will  pass  through 
the  string  of  lamps,  and,  if  all  have  equal  resistances,  there  will  be  the 
same  drop  through  each,  so  that  the  total  voltage  equals  that  around 
one  multiplied  by  the  number  of  lamps.  The  figure  shows  how  this 
method  may  be  used  to  measure  the  voltage  on  an  arc-light  circuit 
when  only  a  I5o-volt  instrument  is  available.  One  can  count  on 
about  50  volts  for  an  "open"  arc,  or  60  volts  to  80  volts  for  an  in- 
closed arc  lamp,  so  that  it  is  safe  to  use  as  many  loo-volt  incandescent 
lamps  in  the  pressure  circuit  as  there  are  arcs  in  the  main  line.  To 
make  sure  that  the  ncandescent  lamps  are  alike,  it  is  well  to  measure 

14*     * •  Arc  Machine  Terminals. > 
_        26-110  volt  lamps  in  series 


f>3n  150-volt 
\L->\  Voltmeter 

FIG.  932.— MEASUREMENT  OF  HIGH  VOLTAGE. 

the  voltage  around  each  one  separately  and  take  the  sum  as  the  total 
voltage.  A  lamp  whose  pressure  is  close  to  the  average  may  then 
be  adopted  for  a  standard,  and  the  voltage  around  that  lamp  multi- 
plied by  the  number  of  lamps  gives  approximately  the  whole  voltage. 
This  method  is  subject  to  two  sources  of  error:  the  total  pressure 
may  vary  during  the  test,  and  the  pressure  around  each  lamp  is 
slightly  less  when  the  voltmeter  is  around  it  than  at  other  times. 
The  latter  error  is  not  more  than  I  per  cent  or  2  per  cent  with  fairly 
good  voltmeters,  which  take  but  little  current. 

933.  How  can  a  voltmeter  be  used  to  measure  a  voltage  lower 
than  its  smallest  reading? 

With  an  instrument  with  two  or  more  scales  (see  No.  847),  the 
measurement  may  sometimes  be  made  by  using  the  lower  scale.  It 
is  sometimes  practicable  to  get  into  the  instrument  and  make  another 


MEASUREMENTS.  239 

connection  so  as  to  obtain  a  scale  lower  than  that  sent,  but  this  in-' 
volves  risk  of  injuring  the  instrument.  When  the  scale  is  open  near 
the  lower  end,  a  small  voltage  may  frequently  be  measured  by  placing 
it  in  series  with  another,  which  will  bring  the  reading  near  the  most 
open  part  of  the  scale.  In  that  case  the  small  voltage  equals  the 
difference  in  the  two  readings,  when  it  does  and  does  not  increase 
the  reading  with  the  auxiliary  voltage  alone.  For  example,  suppose 
it  is  desired  to  measure  the  voltage  of  a  small  battery  when  the  only 
instrument  available  reads  nothing  less  than  15  volts.  Connect  one 
side  of  the  battery  to  one  wire  of  a  dynamo  circuit ;  connect  the  volt- 
meter first  across  the  dynamo  circuit  and  then  across  the  dynamo  and 
battery.  Suppose,  in  the  first  case,  it  reads  106  volts,  and  in  the 
other,  107.8  volts  or  104.2  volts.  The  battery,  therefore,  adds  or 
subtracts  1.8  volts. 

934.  Is  the  above  method  suitable  for  alternating  currents? 

It  is  not  so  reliable  as  with  direct  currents,  for  the  two  alternating 
pressures  are  liable  not  to  be  in  exact  phase,  in  which  case  the  dif- 
ference between  the  two  readings  would  be  too  small. 

935.  What  instruments  are  necessary  for  measuring  resistance? 
High  resistances  may  be  measured  by  a  voltmeter,  by  a  bridge  or 

by  an  ohmmeter.  Low  resistances  may  be  measured  by  an  ammeter 
and  a  voltmeter,  or  by  a  voltmeter  and  standard  resistance.  There 
are  other  methods  suitable  for  laboratories  with  fine  apparatus  and 
skilled  operators,  but  most  measurements  of  resistance  for  practical 
purposes  can  be  made  with  the  apparatus  mentioned. 

936.  How  can  resistance  be  measured  with  a  voltmeter? 

For  this  purpose  a  voltmeter  of  known  resistance  and  a  dynamo 
or  battery  giving  constant  voltage  are  necessary.  First  connect  the 
voltmeter  directly  to  the  terminals  of  the  dynamo,  as  suggested  at  A 
in  the  figure,  or  to  some  circuit  connected  with  it,  on  which  the  cur- 
rent is  not  changing.  Carefully  measure  the  voltage  between  the  lines, 


G 


FIG.  936.— MEASURING  RESISTANCE  WITH  VOLTMETER. 

and  then  connect  in  series  the    resistance    which    is    to    be    meas- 
ured, as  shown  at  B  in  the  figure,  and  again  read  carefully  the  voltage 


240  ELECTRICAL   CATECHISM. 

indicated  on  the  voltmeter.  This  reading  will  be  smaller  than  the 
first,  because  the  resistance  of  the  circuit  is  greater  and  less  current 
goes  through  the  voltmeter  than  before.  Ohm's  law,  that  current 
equals  voltage  divided  by  resistance,  is  equally  true  if  turned  around 
to  read  that  resistance  equals  voltage  divided  by  current.  Now,  when 
the  same  current  flows  through  two  resistances,  the  voltage  or  fall 
of  potential  through  each  is  proportional  to  the  number  of  ohms  re- 
sistance in  each.  In  the  case  above,  the  drop  through  the  voltmeter 
is  evidently  the  reading  of  the  voltmeter,  while  the  difference  be- 
tween that  reading  and  the  first  one  taken  when  the  voltmeter  was 
connected  directly  across  the  circuit  is  the  voltage  drop  through  the 
resistance  being  measured.  This  may  be  expressed  as  follows : 
The  resistance  being  measured  equals  the  resistance  of  the  voltmeter 
multiplied  by  the  difference  between  the  two  readings  and  divided 
by  the  smaller  reading. 

937.  Give  an  example  of  measuring  resistance  by  a  voltmeter. 
Suppose  we  wish  to  measure  the  resistance  of  the  field  coil  of  a 

^-kw,  125-volt  motor.  The  voltmeter  has  a  resistance  of  18,000 
ohms.  When  connected  directly  to  the  circuit,  it  indicates  125  volts; 
when  connected  in  series  with  the  field  coil  of  the  motor,  it  reads  121 
volts.  (Of  course,  in  this  measurement,  it  is  necessary  to  place  the 
voltmeter  a  sufficient  distance  from  the  motor  so  that  its  magnetic 
field  will  not  influence  the  voltmeter).  The  resistance  of  the  motor 
field  is  then  18,000  X  (125  —  121)  -^  121  =  595  ohms. 

938.  How  can  a  voltmeter  be  used  to  measure  insulation  resist- 
ance? 

By  connecting  one  side  of  the  circuit  with  one  side  of  the  dynamo 
and  connecting  the  other  to  it  through  the  voltmeter.  For  example, 
suppose  it  is  desired  to  test  the  insulation  between  the  frame  and  the 
windings  of  the  motor.  Connect  the  frame  of  the  motor  to  one  side 
of  the  dynamo  circuit ;  connect  the  other  side  of  the  dynamo  circuit 
to  one  terminal  of  the  voltmeter,  and  connect  the  other  terminal  of 
the  voltmeter  to  the  armature  or  field  coil  of  the  motor.  Suppose, 
as  before,  that  the  voltage  across  the  dynamo  is  125  volts;  when  the 
voltmeter  is  connected  to  the  motor  as  just  described,  it  reads  2  volts. 
Then  the  resistance  of  the  insulation  is  18,000  X  (125  —  2)  -r-  2  = 
18,000  X  123  -r-  2  =  1,107,000  ohms. 

939.  How  can  the  resistance  of  the  voltmeter  be  determined  f 
The  best  instruments  have  the  resistance  marked  on  the  case  or 

on  the  box.  If  this  is  not  found,  it  can  be  determined  easily  if  one 
can  get  some  high  resistance  of  known  value.  As  before,  first  meas- 


MEASUREMENTS.  241 

ure  the  voltage  across  the  line,  and  then  connect  the  high  resistance 
of  known  value  in  series  with  the  voltmeter.  The  resistance  of  the 
voltmeter  equals  that  of  the  other  multiplied  by  the  second  reading 
and  divided  by  the  difference  between  the  two. 

940.  Give  an  example  of  determining  the  resistance  of  a  voltmeter 
by  means  of  a  known  resistance. 

Suppose  that  the  voltmeter  reads  125  volts  when  connected  di- 
rectly across  the  line,  but  only  75  volts  when  in  series  with  a  resist- 
ance of  10,000  ohms.  The  resistance  of  the  voltmeter  is  then  :  10,000 
X  75  -T-  (125  —  75)  =  750,000  -r-  5°  =  i5»°oo  ohms. 

941.  What  range  of  resistances  can  be  measured  satisfactorily 
with  a  voltmeter? 

It  depends  largely  upon  the  range  and  the  resistance  of  the  volt- 
meter. With  a  Weston  I5o-volt  instrument  one  can  measure  from 
about  100  to  2,500,000  ohms,  the  greatest  sensitiveness  and  accuracy 
being  for  resistances  about  equal  to  that  of  the  voltmeter.  With  one 
having  a  range  from  o  to  5  volts,  one  can  measure  resistances  from 
about  3  to  85,000  ohms. 

942.  How  can  one  measure  resistance  with  an  ammeter  and  a 
voltmeter? 

Connect  the  ammeter  in  series  with  the  resistance  to  be  measured 
and  connect  the  voltmeter  in  shunt  around  it,  as  shown  in  the  figure. 


FIG.  942.— MEASURING   RESISTANCE   BY   AMMETER  AND   VOLT- 
METER. 

By  Ohm's  law,  the  resistance  equals  the  current  divided  by  the  volt- 
age. 

943.  Give  an  example  of  measuring  resistance  by  an  ammeter 
and  a  voltmeter. 

Suppose  the  current  through  one  of  the  field  coils  of  an  arc  dynamo 
is  10  amps.,  while  the  voltmeter  shows  20  volts  when  connected 
around  the  coil.  The  resistance  of  the  coil  is  then :  20  -f-  10  =  2 
ohms. 

The  voltmeter  connected  around  an  arc  lamp  in  the  same  circuit 
shows  45  volts.  The  apparent  resistance  of  the  arc  lamp  (including 
the  arc)  is :  45  -f-  10  =  4.5  ohms. 


242  ELECTRICAL   CATECHISM. 

944.  What  precautions  are  necessary  to  insure  accuracy  when 
measuring  resistance  with  a  voltmeter? 

Be  careful  that  the  voltage  of  the  dynamo  or  battery  does  not 
change  between  the  two  readings;  this  can  usually  be  checked  by 
taking  a  second  reading  across  the  line  with  the  voltmeter  alone. 
Be  sure  also  that  the  voltmeter  is  not  affected  by  any  magnetic  field 
from  the  circuit  being  measured.  Also  be  sure  that  the  wires  and 
connections  used  do  not  have  any  appreciable  resistance,  or  that 
must  be  considered. 

945.  What  precaution  should  be  taken  when  measuring  with  am- 
meter and  voltmeter? 

Be  careful  that  neither  instrument  is  affected  by  magnetic  field 
from  the  other  instrument  or  from  any  other  source.  Be  careful  not 
to  use  more  current  than  is  suitable  for  the  resistance  being  meas- 
ured. Be  careful  about  the  connections,  especially  those  at  the 
terminals  of  the  resistance  which  are  to  be  measured,  for  if  these  are 
not  good  the  voltmeter  will  read  too  high.  There  is  a  small  source 
of  error  in  the  fact  that  if  the  voltmeter  is  connected  inside  the  am- 
meter, the  ammeter  measures  the  current  through  the  voltmeter 
as  well  as  that  taken  by  the  resistance ;  on  the  other  hand,  if  the  volt- 
meter is  connected  behind  the  ammeter  it  measures  not  only  the  drop 
through  the  resistance,  but  also  that  through  the  ammeter,  and 
through  the  connections.  It  is  usually  better  to  neglect  the  current 
in  the  voltmeter  and  connect  it  directly  to  the  terminals  of  the  re- 
sistance. Suitable  corrections  can  be  made  without  great  difficulty. 

946.  What  is  an  ohmmeter? 

This  is  an  instrument  sometimes  used  for  measuring  resistance 
where  it  is  desirable  to  test  with  a  high  voltage,  but  where  none  is 
available  except  as  supplied  by  the  testing  outfit.  It  consists  of  two 


FIG.   946.— OHMMETER. 

parts,  a  small  hand  dynamo  capable  of  generating  100  volts  or  more. 
and  the  instrument  proper.  The  latter  has  two  coils  at  right  angles 
to  each  other,  and  a  magnetic  needle,  which  takes  a  position  between 
the  two  coils  according  to  the  relative  currents  in  the  coils  One 
coil  is  connected  between  the  dynamo  and  one  terminal  of  the  circuit 
whose  resistance  is  to  be  measured ;  the  other  coil  is  connected  in 
series  with  a  high  resistance  inside  the  instrument  so  as  to  form  a 


MEASUREMENTS.  243 

shunt  across  the  main  circuit,  as  shown  in  the  figure.  When  the 
dynamo  is  operated,  the  current  divides,  part  going  through  the  coil, 
C,  and  then  through  the  main  circuit,  whose  resistance  is  to  be  meas- 
ured, while  the  remainder  goes  through  the  coil,  P,  and  the  high 
resistance,  R.  The  currents  through  the  two  coils  are  inversely  pro- 
portional to  the  resistances  in  their  circuits,  since  the  same  voltage  is 
applied  to  both,  and  therefore  they  attract  the  needle  correspond- 
ingly. For  a  given  voltage  of  the  dynamo,  the  attraction  of  the  coil, 
P,  is  constant,  while  the  attraction  of  coil,  C,  becomes  greater  as  the 
resistance  in  the  main  line  becomes  less.  The  pointer  attached  to 
the  needle  will  therefore  move  toward  C  as  the  resistance  in  the  line 
becomes  less,  and  the  scale  may  be  divided  to  indicate  the  resistance 
in  the  main  circuit. 

947.  For  what  range  of  resistance  is  the  ohmmeter  suitable? 

It  will  measure  resistances  from  about  5  megohms  (5,000,000 
ohms)  down  to  about  1000  ohms.  It  is  convenient  for  measuring  the 
insulation  resistance  of  wiring  in  houses  that  are  not  yet  connected 
with  the  supply  circuit. 

948.  What  is  a  Wheatstone  bridge? 

The  Wheatstone  bridge  is  an  arrangement  of  resistances  with  a 
battery  and  galvanometer  for  measuring  resistances.  It  usually  con- 


FIG.  94SA.— WHEATSTONE  BRIDGE. 

sists  of  two  "proportional  arms"  of  such  values  that  one  arm  has 
one,  ten,  one  hundred  or  one  thousand  times  the  resistance  of  the 


244 


ELECTRICAL   CATECHISM. 


other  arm.  The  third  arm  is  divided  into  tenths,  units,  tens,  hun- 
dreds and  thousands  of  ohms.  The  resistance  to  be  measured  be- 
comes the  fourth  arm.  When  the  bridge  is  balanced,  the  unknown 
resistance  has  the  same  ratio  to  that  of  the  divided  arm  as  one  of  the 
proportional  arms  has  to  the  other. 


FIG.   948B.-WHEATSTONE  BRIDGE. 


949.     W/iaf  is  the  theory  of  the  bridge? 

Suppose  we  have  two  conductors  connected  in  multiple,  and  hav- 
ing a  battery  or  other  source  of  current  applied  to  their  terminals. 
The  potential  would  fall  from  one  end  to  the  other  in  each  wire,  and 
for  any  point  in  one  wire  a  corresponding  point  in  the  other  can  be 
found  at  which  the  potential  has  fallen  the  same  amount.  If  a 
galvanometer  is  connected  between  these  two  points,  no  current  will 
flow  since  there  is  no  difference  of  potential  between  its  terminals. 
We  have  a  close  analogy  in  the  case  of  a  river  which  divides  and 
flows  around  an  island ;  the  water  at  every  point  along  a  line  across 
the  river  at  the  head  of  the  island  is  at  the  same  level;  also  every 
point  in  another  line  across  the  river  at  the  lower  end  of  the  island 
is  at  the  same  level ;  if  the  water  flows  along  the  two  sides  of  the 
island  without  any  sudden  falls,  for  any  point  on  one  side  of  the 
island,  we  may  find  a  corresponding  point  on  the  other  side  which 
has  the  same  level ;  between  these  two  points  no  current  would  flow 
if  a  canal  were  dug  between  the  two.  In  the  Wheatstone  bridge 
the  galvanometer  circuit  corresponds  to  the  canal  across  the  island, 
and  when  no  current  flows  through  the  galvanometer  it  shows  that 
the  two  ends  of  the  circuit  have  the  same  potential.  If  then  we  know 
that  one  end  of  the  canal  is  one-tenth  of  the  total  fall  from  the  upper 
to  the  lower  level,  we  are  sure  that  the  other  end  is  at  a  corresponding 
point. 


MEASUREMENTS. 


245 


950.     Give  an  example  illustrating  the  principle  of  the  bridge. 
The  principle  of  the  bridge  may  be  explained  by  assuming  resist- 
ances connected  as  in  Fig.  95OA  with  battery  giving  1 1  volts  between 
A  and  C.     The  currents  in  the  various  circuits  are  determined  by 
Ohm's  law : 
Current  in  circuit  ABC  =  I'  =  E/R'  =  u/(45  +  Jo)  =  "/55  — 

0.2  ampere ; 

Current  in  circuit  ADC  =  I"  =  E/R"  =  n/(9O  +  20)  =  n/iio  = 

o.i  ampere. 


no  volts 
FIG.  950A.— PRINCIPLE  OF  BRIDGE.  FIG.  950B.— WHEATSTONE  BRIDGE. 

The  fall  of  potential  or  "  ohmic  drop  "  in  parts  AB  and  AD  is : 
Drop  A  to  B  =  s  e'  =  I'r'  =  0.2  X  45  =  9  volts ; 
Drop  A  to  D  =  s  e"  =  IV  =  o.i  X  90  =  9  volts. 
Since  drops  A-B   and   A-D   are   equal,   there   is   no   difference   in 
potential  between  B  and  D,  hence  no  current  flows  through  the  gal- 
vanometer G.     Similar  calculations  would  give  a  balance  for  CB 
and  CD.     For  this  condition  of  balance, 

aXd=cX*,or  x  =  a  X  d/c. 

In  practice,  the  resistances  of  the  two  wires,  c  and  d,  are  fixed  in  a 
certain  ratio  for  a  given  measurement  (in  this  case,  20  190,  but 
usually  as  10:  100  or  a  multiple  of  10),  and  the  value  of  a  is  varied 
until  a  balance  is  obtained.  A  commercial  Wheatstone  bridge  is 
shown  herewith,  with  the  parts  lettered  to  correspond  to  the  above, 
x  being  the  resistance  under  measurement.  In  this  case,  c  is  the 
left  part  of  the  upper  row,  d  the  right  part,  and  the  two  lower  rows 
correspond  to  a.  If  c  were  plugged  for  1000,  and  d  for  10,  and  if 
1425  plugged  in  b  brought  a  balance,  then  the  value  of  x  won1  i  be 
14.25  ohms. 

951.     Is  it  safe  to  use  a  bridge  on  a  no-volt  circuit? 

Such  a  voltage  would  send  too  much  current  through  the  bridge  and 
damage  it.  It  is  customary  to  use  a  battery  giving  2  to  4  volts,  al- 
though in  measuring  high  resistances,  it  is  sometimes  desirable  to 
use  higher  voltages.  The  current  should  be  kept  on  only  a  very 


246  ELECTRICAL  CATECHISM. 

short  time,  in  order  to  avoid  heating  the  bridge  and  so  changing  its 
resistances. 

952.  What  range  of  resistances  can  be  measured  conveniently 
with  a  bridge? 

It  depends  largely  upon  the  bridge  and  the  galvanometer.  The 
best  bridges  will  measure  from  o.oooooi  to  111,000,000  ohms.  Or- 
dinary portable  bridges  will  measure  from  o.oi  to  1,100,000  ohms 
with  fair  accuracy. 

953.  Are  bridges  suitable  for  testing  the  resistances  of  armatures 
of  large  dynamos? 

They  are  not  usually  considered  accurate  for  such  purposes,  since 
the  resistance  when  running  is  apt  to  be  different  from  that  when 
standing  still ;  such  resistances  should  always  be  measured  when  the 
machine  is  hot  and  carrying  current,  and  for  such  work  an  ammeter 
and  a  low  reading  voltmeter  are  more  reliable. 

954.  Is  a  bridge  suitable  for  testing  insulation  resistance? 

Not  if  the  insulation  is  to  be  used  for  high  potentials.  The  reasons 
for  this  are  two :  Insulation  for  high  potentials  should  be  tested  not 
so  much  for  ohmic  resistance  as  for  its  ability  to  stand  the  high 
potentials;  again,  the  resistance  with  a  low  potential  may  be  quite 
different  from  that  with  a  high  potential,  on  account  of  the  electro- 
static attraction  between  different  parts  bringing  them  closer  to- 
gether. 

955.  How  can  a  circuit  be  tested  by  a  magneto ? 

By  connecting  the  magneto  to  the  circuit  and  turning  the  crank; 
if  the  resistance  is  not  too  high,  or  if  the  circuit  has  enough  electro- 
static capacity,  the  bell  will  ring.  (See  Nos.  115,  1125  to  1131.) 

956.  How  can  resistance  be  measured  by  means  of  a  standard  re- 
sistance and  voltmeter? 

The  circuit  whose  resistance  is  to  be  measured  is  connected  in 
series  with  a  standard  resistance,  that  is,  a  conductor  whose  re- 
sistance is  already  known,  and  a  steady  current  is  sent  through  both. 
The  voltmeter  is  connected  first  around  one  resistance  and  then 
around  the  other,  as  suggested  in  the  figure.  Since  the  same  current 
flows  through  both  conductors,  the  voltage  or  fall  of  potential 
through  each  is  proportional  to  its  resistance,  and  we  may  write  the 
equation :  current  equals  voltage  around  standard  divided  by  its  re- 
sistance; also  equals  voltage  around  conductor  being  measured  di- 
vided by  its  resistance,  or 

Ep 
,  ,  _.       s *^X 

~~' 


MEASUREMENTS. 


247 


From  this  we  get  :  Resistance  of  conductor  being  measured  equals 
the  voltage  around  it  multiplied  by  the  resistance  of  the  standard 
and  divided  by  the  voltage  around  the  standard,  or 


_ 

x  - 


F  R 

^     lx 


V  J I  AM£X-£LEC*  I     V     V 

* — S  ^^* 

VOLTMETER  VOLTMETER 

FIG.  961.— MEASURING  RESISTANCE  BY  FALL  OF  POTENTIAL* 

957'  Give  an  example-  of  measuring  resistance  by  use  of  volt- 
meter  and  standard  resistance. 

Suppose  it  is  desired  to  measure  the  resistance  of  a  4-kw  dynamo. 
A  standard  resistance  of  o.oi  ohm  is  connected  in  series  with  the 
armature  and  a  current  of,  say,  24  amps,  is  sent  through  both.  The 
voltmeter  shows  0.24  volt  when  connected  around  the  standard.  The 
terminals  of  the  voltmeter  are  then  pressed  against  the  same  com- 
mutator bars,  against  which  the  brushes  rest  (the  armature  being 
held  stationary)  and  the  drop  through  the  armature  is  measured ;  the 
armature  is  then  turned  to  a  new  position  and  the  voltage  is  again 
measured  between  the  commutator  bars  under  the  brushes;  this  is 
repeated  for  several  positions.  Suppose  the  average  is  5.0  volts. 
The  resistance  of  the  armature  is  then : 

ExRs      5.0x0.01  , 

Rx  =    T..    s  —  —    -^r—  =  o.  208  ohms. 
Ls  o.  24 

958.  What  precautions  are  necessary  for  accuracy  in  measuring 
resistance  by  standard  resistance  and  voltmeter? 

The  standard  resistance  must  be  suitable  for  carrying  current  of 
considerable  magnitude  without  appreciable  heating ;  ordinary  stand- 
ard resistances  consist  of  fine  wire  wound  closely  on  spools  with 


248  ELECTRICAL  CATECHISM. 

little  chance  for  cooling,  and  such  can  safely  carry  only  smsrll  cur- 
rents without  undue  rise  of  temperature.  The  "shunts"  used  with 
Weston  switchboard  ammeters  are  suitable  for  such  use.  One 
should  be  careful  that  the  current  does  not  change  while  the  voltages 
are  being  measured.  The  voltmeter  should  have  fairly  high  re- 
sistance so  as  not  to  divert  an  appreciable  amount  of  current  from 
the  conductor  around  which  it  is  shunted.  Of  course  the  voltmeter 
should  give  correct  readings  and  the  standard  resistance  should  be 
reliable.  The  voltmeter  should  be  shunted  around  the  resistance  to 
be  measured,  care  being  taken  not  to  include  the  connections  or  any 
other  resistance.  The  voltmeter  should  be  suitable  for  small  voltages. 

959.  How  can  a  ground  on  an  arc  circuit  be  located  when  the 
lamps  are  lighted,  using  a  bank  of  incandescent  lamps? 

Connect  in  series  by  bare  wires  two  strings  of  incandescent  lamps, 
each  having  half  as  many  as  the  total  number  of  arcs  in  series,  using 
no-volt  lamps  to  represent  "  open  "  or  5o-volt  arcs,  or  22O-volt  (or 
twice  as  many  no-volt)  lamps  to  represent  enclosed  arcs;  connect 
each  dynamo  terminal  to  one  end  of  a  string  through  a  safe  switch ; 
connect  the  other  ends  of  the  lamp  strings  to  switches  by  which 
either  or  both  may  be  grounded ;  to  the  ground  connect  one  end  of 
an  insulated  No.  14  cable  long  enough  to  reach  the  length  of  each 
string  of  incandescents,  and  having  a  terminal  wire  tip  with  an  in- 
sulated handle.  To  test  the  circuit,  connect  one  lamp  string  to  the 
ground ;  if  the  lamps  do  not  glow,  disconnect  that  string  and  ground 
the  other ;  if  neither  string  glows  when  grounded  alone,  the  line  is  free 
from  grounds.  If  one  or  both  strings  glow,  ground  both  of  them,  when 
one  string  will  glow  more  brightly  than  the  other ;  beginning  at  the 
grounded  center,  touch  the  cable  tip  to  one  lamp  junction  after  the 
other,  until  a  place  is  found  where  the  lamps  (except  those  between 
the  tip  and  the  ground)  are  equally  bright ;  the  number  of  lamps  be- 
tween the  tip  and  the  dynamo  terminal  then  represents  the  number 
of  arc  lamps  between  that  terminal  and  the  ground. 


CHAPTER  IX. 


ELEMENTARY  MOTORS. 

rooo.  What  are  'the  fundamental  principles  upon  which  the  elec- 
tric motor  operates? 

Every  current  is  surrounded  by  a  magnetic  field  of  force  which 
distorts  any  other  magnetic  field  in  the  vicinity ;  the  two  combine 
into  a  resultant  field  whose  lines  of  force  tend  to  become  as  short  as 
possible,  thereby  tending  to  move  both  sources  to  a  new  arrange- 
ment, such  that  the  lines  may  be  as  short  as  possible.  In  accom- 
plishing this  result,  what  is  called  the  "field"  is  usually  the  space  be- 
tween the  poles  of  an  electromagnet ;  the  current  is  in  wires  carried 
on  or  near  the  surface  of  a  cylinder  of  laminated  iron,  the  combina- 
tion of  iron  and  wire  being  called  the  "armature." 

1001.  Are  alternating-current  motors  based  upon  the  same  prin- 
ciples as  those  for  direct  currents? 

The  fundamental  principles  are  the  same,  although  in  the  applica- 
tion there  are  many  differences.  With  direct-current  motors  the  cur- 
rents for  both  the  field  and  the  armature  are  taken  from  the  external 
circuit ;  with  most  of  the  motors  for  alternating  currents  the  field  is 
magnetized  by  current  taken  from  the  external  circuit,  while  the  cur- 
rent in  what  corresponds  to  the  armature  is  often  obtained  by  induc- 
tion. 

1002.  Will  an  alternating-current    motor  work    upon  a  direct- 
current  circuit? 

A  few  small  alternating-current  motors  will  work  on  a  direct  cur- 
rent, but  most  of  the  larger  ones  will  not. 

1003.  Will  a  direct-current  motor  work  upon  an  alternating  cur- 
rent? 

Almost  any  direct-current  motor  will  run  when  an  alternating 
current  is  sent  through  it ;  but  it  will  not  ordinarily  develop  any  con- 
siderable amount  of  power,  unless  wound  specially  for  the  purpose, 
and  unless  the  field  magnets  are  made  of  laminated  iron  and  pre- 
cautions are  taken  to  avoid  induced  currents  in  stationary  parts  of 
the  motor. 


250  ELECTRICAL  CATECHISM. 

1004.     W  hat  is  the  simplest  form  of  motor? 

Those  in  which  the  wires  or  iron  parts  change  their  position  under 
the  action  of  the  current  so  as  to  reduce  the  reluctance  of  the  mag- 
netic circuit.  Such  elementary  motors  generally  have  a  movable 
piece  of  soft  iron,  called  a  "keeper"  or  "armature,"  which  is  attracted 
against  gravity  or  a  spring  by  the  action  of  the  current,  and  moves 
closer  to  the  poles  of  the  magnet  so  as  to  improve  the  magnetic  cir- 
cuit. In  some  cases  the  action  of  the  current  in  the  coil  is  to  lengthen 
the  path  of  the  lines  of  force,  and  their  tendency  to  shorten  brings 
the  coil  into  a  new  position,  as  in  the  instruments  of  the  d'Arsonval 
type  (see  Nos.  833  to  836).  Many  of  the  measuring  instruments 
(see  Nos.  822  to  840)  are  elementary  motors. 


1005.  What  are  common  examples  of  simple  motors  based  on 
the  attraction  between  a  magnet  and  piece  of  iron? 

The  telegraph  sounder  or  relay,  the  telephone  receiver  and  the 
electric  bell  are  familiar  examples. 

1006.  Explain  the  action  of  the  telegraph  sounder. 

The  current  passes  through  the  two  coils,  magnetizing  the  iron 
core  and  attracting  the  soft  iron  armature  or  keeper.  When  the 
current  stops  for  an  instant,  the  magnetic  pull  becomes  less  and  the 
spring  pulls  the  armature  away.  The  current  is  interrupted  by  keys, 
by  which  the  operator  closes  and  opens  the  circuit  and  so  causes 
the  magnets  to  attract  the  armatures  longer  or  shorter  times.  In 
the  receiving  instrument,  commonly  used,  the  message  is  read  from 
the  noise  made  by  the  armature  as  the  lever  or  hammer  strikes  upon 
the  anvil  or  adjusting  screws.  The  "dots  and  dashes"  of  the  re- 
cording instruments  are  distinguished  by  the  longer  or  shorter 
sounds  of  the  armature,  the  sound  of  the  "dashes"  being  somewhat 
louder  that  that  for  the  "dots,"  as  the  current  is  on  longer  and  mag- 
n^tizes  the  iron  more  strongly.  The  circuit  for  a  short  line  is 


.T 

:nV 


i 

-L-  EARTH  BARXb  —  ^ 

FIG.  1006.-TELEGRAPH  CIRCUIT. 

indicated  by  the  figure.  When  the  line  is  a  long  one  connecting  a 
number  of  stations,  it  is  customary  to  use  relays  in  connection  with 
the  sounders,  the  relays,  keys,  line  and  batteries,  forming  the  circuit 
with  the  earth  as  the  return,  as  indicated  by  Fig.  1006. 


ELEMENTARY  MOTORS. 


251 


1007.     Explain  the  relay. 

The  relay  is  a  special  form  of  telegraph  receiving  instrument.  The 
resistance  of  long  lines  with  their  numerous  instruments  and  long 
wire  is  so  large  that  it  would  be  almost  impossible  to  send  enough 
current  to  work  the  sounders  with  sufficient  force  to  be  heard  dis- 


FIG.  1007.— TELEGRAPH  CIRCUIT. 

tinctly  if  there  was  any  disturbing  noise.  In  order  to  make  the  sig- 
nals intelligible,  even  in  noisy  offices,  it  is  customary  to  have  a  relay 
in  the  main  line.  The  current  is  strong  enough  to  move  the  arma- 
ture back  and  forth  to  close  or  open  the  "local  circuit,"  which  con- 
sists of  a  battery  and  sounder,  as  indicated  in  the  figure.  In  this 
way  the  noise  is  multiplied.  The  relay  magnets  are  wound  with 
many  turns  of  fine  wire  to  a  resistance  of  30  to  300  ohms,  while 
sounders  have  from  10  to  50  ohms. 


1008.     How  are  recording  telegraph  receivers  made? 

These  carry  a  strip  of  paper  which  passes  over  an  ink  roller  or  a 
sharp  point,  so  that  at  each  stroke  of  the  armature  a  long  or  short 
mark  is  made  on  the  paper.  The  figure  shows  a  common  type  of  ink- 
writing  register,  such  as  used  in  fire  and  district  telegraph  offices. 
These  have  a  clockwork  mechanism  for  feeding  the  paper,  and 
usually  have  attachments  for  starting  and  stopping  the  paper,  so  that 
it  feeds  only  when  the  receiver  is  operating.  In  the  case  of  ocean 
cables,  the  current  is  so  very  weak  that  a  specially  sensitive  receiver 


252 


ELECTRICAL  CATECHISM. 


is  arranged  so  as  to  operate  with  very  little  friction,  the  ink  being 
squirted  against  the  paper  by  electrostatic  repulsion  (see  No.  91) 
or  jolted  against  it  by  an  electromagnetic  vibrator. 


FIG.  1008A.— INK  WRITING  REGISTER. 


FIG.  1009A.— FIRE  ALARM  CIRCUIT. 


1009.     Hozv  are  fire  alarms  operated? 

The  fire  alarm  system  is  practically  a  telegraph  circuit,  in  which 


FIG.  1009B.-FIRE-ALARM  SIGNAL  BOXES. 


ELEMENTARY  MOTORS. 


253 


the  keys  are  replaced  by  notched  wheels  so  arranged  as  to  break  and 
make  the  circuit  some  definite  number  of  times,  a  weight  or  spring 
in  the  alarm  box  causing  the  wheel  to  revolve  whenever  the  door 
is  opened,  and  the  lever  is  pulled  down  once  and  let  go.  At  the 
central  office  of  the  fire  department  is  a  relay  in  each  circuit,  arranged 
to  repeat  the  alarm  at  each  station  house,  where  another  relay 
operates  the  gongs  and  rings  the  tower  bell.  A  simple  alarm  circuit 
is  shown  in  the  figure. 

1010.     Explain  the  principle  of  the  electric  bell. 

An  electric  bell  outfit  consists  of  four  essential  parts,  the  bell,  the 
battery,  the  push-button  or  switch,  and  the  connecting  wire.  When 
the  button  is  pushed  the  circuit  is  closed,  and  current  from  the  bat- 


FIG.  1010-1.— ELECTRIC  BELL 
CIRCUIT. 


FIG.  1010-2.-ELECTRIC  BELL. 


tery  passes  through  the  circuit.  In  the  bell  it  passes  through  the  coils 
of  an  electromagnet,  thereby  magnetizing  its  soft  iron  core.  This 
attracts  the  soft  iron  armature  and  draws  it  closer  to  the  coils,  and 
thus  the  hammer  at  the  end  of  the  armature  strikes  the  bell.  But 


254  ELECTRICAL   CATECHISM. 

when  the  armature  is  drawn  up,  the  spring  behind  it  is  drawn  away 
from  the  contact  screw,  and  thus  opens  the  circuit  and  stops  the 
current.  This  causes  the  iron  core  to  lose  its  magnetism,  so  that  it 
can  no  longer  hold  the  armature  against  the  pull  of  the  tension 
spring.  The  armature,  therefore,  falls  back  until  its  spring  touches 
the  contact  screw,  when  the  circuit  is  closed  again,  and  the  same 
action  is  repeated.  Thus  the  armature  oscillates  back  and  forth, 
opening  and  closing  the  circuit  automatically,  and  we  have  an  ele- 
mentary motor,  which  operates  so  long  as  the  circuit  is  closed  at 
the  push-button  or  switch.  The  principal  parts  of  the  bell  are  shown 
in  Fig.  2. 

ion.     What  is  a  "single  stroke"  electric  bell? 

The  vibrating  contact  is  removed  so  that  there  is  no  automatic 
make-and-break  in  the  bell  itself.  The  armature  is  therefore  at- 
tracted only  once,  and  strikes  the  gong  one  blow  each  time  the  circuit 
is  closed  at  the  switch.  It  is  thus  suitable  for  giving  signals. 

1012.     What  is  an  electro-mechanical  bell? 

Electro-mechanical  bells  are  usually  single  stroke.  When  the 
magnet  attracts  its  armature,  this  releases  some  sort  of  a  catch  or 
trigger  that  sets  free  a  train  of  clockwork  which  strikes  the  gong 


FIG.  1012.— ELECTRO-MECHANICAL  GONG. 

a  harder  blow  than  would  be  practicable  with  the  magnet  alone.  The 
electromagnet  thus  acts  as  a  sort  of  relay  or  starter,  while  the  blow 
on  the  gong  is  struck  by  a  mechanism  driven  by  a  spring  or  weight. 


ELEMENTARY  MOTORS. 


255 


1013.  What  i$  a  buzzer? 

A  buzzer  is  electrically  the  same  as  an  ordinary  electric  bell,  ex- 
cept that  there  is  no  gong.  The  armature  vibrates  rapidly  and  makes 
a  buzzing  noise  that  is  sufficient  to  attract  the  attention  of  a  person 
near  by. 

1014.  How  does  the  telephone  receiver  operate? 

The  telephone  receiver  is  an  elementary  motor  operated  by  an 
electric  current  which  very  rapidly  changes  its  strength  and  usually 
its  direction.  The  air  is  thrown  into  sound  waves  by  the  vibration 
of  a  thin  iron  diaphragm  which  is  attracted  with  varying  strength 
by  one  or  both  poles  of  a  permanent  magnet.  The  pulsating  current 
from  the  telephone  transmitter  (see  No.  347)  is  usually  sent  through 
an  induction  coil  or  transformer  (see  Nos.  1404  and  1405)  which 
delivers  an  alternating 

current  of  high  voltage,  ^BNNNNNNN:^^T^mrpJ^-ear  p*ece 

suitable  for  passing  over 
long  lines ;  this  current  A 
in  going  through  the  coil  O 
of  the  telephone  receiver 
increases  and  decreases 
the  proportion  of  mag- 
netism from  the  perma- 
nent magnet  through 
the  pole  tips  to  the  dia- 
phragm, so  that  the  lat- 
ter is  attracted  more  or 
less  strongly;  it  there- 
fore vibrates  in  unison 
with  every  pulsation  of 
the  current,  and  hence 
in  unison  with  the  tele- 
phone transmitter  and 
with  the  voice. 

1015.  How  does  the  magneto  bell  operate? 

The  magneto  bell,  sometimes  called  a  ringer,  operates  by  the  action 
of  an  alternating  current  on  a  pair  of  iron  cores  and  vibrator  which 
form  parts  of  the  circuit  of  a  permanent  magnet.  When  the  cur- 
rent is  in  one  direction,  the  magnetism  in  one  of  the  cores  is  strength- 
ened and  that  in  the  other  is  weakened  or  reversed ;  the  vibrator  moves 
in  one  direction  or  the  other,  so  as  to  carry  the  magnetic  lines  from 
the  pole  of  the  permanent  magnet  through  the  core  which  at  that 


soft  iron  pole  piece 


—  compound  bar  magnet 


t-hard  rubber  shell 
leading-in  wire 

screw 
-iron  end  piece 

tail  piece 
binding  post 


FIG.   10H.— TELEPHONE    RECEIVERS. 


256 


ELECTRICAL  CATECHISM. 


moment  is  magnetized  by  the  current  in  the  same  direction  as  by  the 
permanent  magnet;  when  the  direction  of  the  current  changes,  the 
vibrator  is  attracted  toward  the  other  core.  In  the  magneto  bell, 
the  vibrator  carries  an  arm  and  hammer  which  strikes  one  of  two 
bells  at  each  change  in  direction  of  the  current. 

A  magneto  bell  may  be  arranged  for  "  selective  ringing  "  by  at- 


Hammer 


Permanent" 
frfcigne.1' 


Elecfro  magnet 


FIG.  1015.— MAGNETO    RINGER. 

taching  a  light  spring  to  the  vibrator  so  as  to  hold  it  normally  against 
one  of  the  bells;  pulsating  currents  (halves  of  alternating  currents, 
see  No.  1401),  are  used  for  selective  ringing;  bells  "  biased  "  by 
the  spring  on  the  positive  side  are  unable  to  respond  to  positive 
impulses,  since  their  vibrators  are  already  on  that  side  with  the 
hammer  against  the  bell,  but  they  will  respond  to  negative  impulses ; 
likewise,  bells  biased  in  the  opposite  direction  will  respond  only  to 
positive  impulses;  by  adjusting  the  natural  period  of  the  bell,  it  may 
be  made  to  respond  to  impulses  of  one  frequency  but  not  to  those  of 
another,  thus  making  possible  the  use  of  several  frequencies;  for 
example,  eight  biased  tuned  bells  might  be  connected  to  one  circuit 
to  which  positive  or  negative  impulses  might  be  applied  at  any  one  of 
say  four  frequencies  (see  No.  1435),  and  yet  only  the  desired  one 
would  ring. 

1016.     How  does  a  pole-changer  operate? 

A  pole-changer  is  a  device  for  obtaining  alternating  or  pulsating 
currents  from  a  battery  or  other  source  of  direct  current,  for  the 
purpose  of  ringing  telephone  bells.  The  details  vary  in  different 
types,  but  the  general  principle  is  that  of  a  vibrating  bell  (see  No. 
1010)  whose  armature  opens  and  closes  a  number  of  contacts  which 
are  essentially  a  reversing  switch.  By  this  means  each  terminal 
of  a  line  is  connected  alternately  with  the  positive  and  negative  termi- 


ELEMENTARY  MOTORS. 


257 


nals  of  a  battery  for  obtaining  alternating  currents;  or  the  line 
terminals  are  connected  intermittently  with  the  same  battery  termi- 
nals to  obtain  pulsating  current  for  selective  ringing  (see  No.  1015). 
Some  instruments  have  an  adjustable  weight  on  the  vibrator,  by 


FIG.  1016.— POLE    CHANGERS. 


which  its  period  and  the  number  of  impulses  or  alternations  per 
second  may  be  adjusted. 

1017.     How  does  an  electric  counter  operate? 

The  armature  of  an  electromagnet  is  attached  to  a  pawl   and 


FIG.  1017.— ELECTRIC    COUNTER. 


ratchet  which  drive  a  counting  train  of  gears.  The  pawl  turns  the 
ratchet  wheel  one  notch  each  time  current  is  sent  through  the  mag- 
net coils. 


CHAPTER  X. 

DYNAMOS. 

(Direct  Current*) 

noo.     What  are  the  principal  sources  of  electrical  currents? 
Dynamos    (often  called  generators),  chemical  cells  or  batteries 
(see  Nos.  601  to  636)  and  thermopiles  (see  Nos.  400  to  406.) 

noi.     What  is  a  dynamo? 

A  dynamo  is  a  machine  for  changing  mechanical  power  into  elec- 
trical power. 

1102.  What  is  the  difference  between  dynamo-electric  machines, 
dynamos  and  generators? 

The  word  "dynamo"  is  simply  a  shorter  term  for  "dynamo-electric 
machine."  The  word  "generator"  is  more  general  and  may  refer 
to  a  dynamo  or  to  a  battery,  or  any  other  source  of  electricity.  It  is 
common  to  use  the  word  "dynamo"  when  the  current  is  used  for 
operating  electric  lights,  and  the  word  "generator"  when  the  cur- 
rent is  used  for  motors.  This  usage  is  simply  a  custom. 

1103.  For  what  purposes  are  dynamos  used? 

For  any  purpose  requiring  any  considerable  amount  of  energy, 
such  as  electric  lights,  motors,  welding,  etc. 

1104.  Are  dynamos  cheaper  than  batteries? 

It  depends  upon  the  amount  of  current  needed.  Where  only  small 
amounts  are  used,  batteries  are  cheaper.  But  if  the  work  requires 
as  much  as  a  quarter  of  a  horse-power,  or  perhaps  less,  it  is  cheaper 
and  easier  to  use  a  dynamo  driven  by  steam,  water,  or  any  other 
convenient  power. 

1105.  How  is  electricity  generated  in  a  dynamo? 

The  exact  action  is  not  entirely  understood,  although  enough  is 
known  about  it  to  construct  working  theories  and  to  design  machines 
with  accurate  and  positive  knowledge  of  results.  The  fundamental 
principles  are  that  an  electromotive  force  is  induced  by  every  change 
in  the  number  of  lines  of  magnetic  force  enclosed  by  a  circuit,  and 
that  an  electric  current  is  surrounded  by  a  magnetic  field. 


DYNAMOS.  259 

1106.  //  the  presence  of  electric  current  produces  a  magnetic 
field,  does  the  presence  of  a  magnetic  Held  produce  electricity? 

The  mere  presence  of  a  magnetic  field  around  a  conductor  does  not 
produce  electricity,  since  that  would  imply  the  actual  creation  of 
energy,  which  is  impossible.  It  is  true,  however,  that  the  movement 
of  a  conductor  across  a  magnetic  field,  or  of  a  magnetic  field  across 
a  conductor,  always  causes  a  tendency  for  current  to  flow  in  the 
conductor.  If  the  conductor  forms  a  closed  or  complete  circuit,  a 
current  will  flow  in  it. 

1107.  Is  there  any  special  name  for  such  a  tendency  for  current 
to  How? 

It  is  called  an  induced  electromotive  force,  or  E.M.F. 

1108.  How  can  one  study  the  induction  of  currents? 

Much  can  be  learned  by  simple  experiments  with  galvanometer 
and  coil  of  wire. 

1109.  What  is  a  galvanometer? 

The  galvanomenter  is  an  instrument  for  detecting  or  measuring 
small  currents.  A  convenient  one  may  be  made  by  wrapping  a 
number  of  turns  of  wire  around  a  pocket  compass  (say  40  turns  of 
No.  24  cotton-covered  wire)  and  placing  it  level,  and  so  that  the 
needle  lies  in  the  plane  of  the  coil.  The  compass  needle  may  be  in- 


FIG.  1109.— DETECTOR  GALVANOMETER. 

side  the  coil  or  may  be  above  it,  as  in  the  "detector  galvanometer" 
illustrated.  Such  a  crude  instrument  can  not  compare  with  the  ex- 
pensive ones  in  laboratories,  but  it  will  do  for  some  purposes.  (See 
No.  834.) 

I  no.     How  can  induction  be  studied  with  galvanometer? 

Make  a  coil  2  ins.  or  3  ins.  in  diameter  with  forty  or  more  turns 
of  insulated  copper  wire  of  any  convenient  size.  Connect  the  ends 
of  the  coil  with  the  terminal  wires  of  a  galvanometer  as  suggested 
in  the  figure.  Slip  the  coil  quickly  over  the  end  of  a  magnetized 


260 


ELECTRICAL  CATECHISM. 


screwdriver  or  any  other  magnet  and  the  needle  of  the  galvano- 
meter will  be  thrown  to  one  side,  say  to  the  right,  and  will-then  return 
to  its  former  position.  Move  the  coil  quickly  off  the  screwdriver 


0 


FIG.  1110.— INDUCTION. 

and  the  needle  will  be  thrown  in  the  opposite  direction,  to  the  left. 
Twist  the  coil  half-way  around  and  slip  it  over  the  end  of  the  magnet 
and  the  needle  is  thrown  to  the  left.  Slip  the  coil  clear  over  and  off 
at  the  other  end,  and  the  needle  is  thrown  in  one  direction  at  one 
end,  but  in  the  opposite  direction  at  the  other  end.  Hold  the  coil 
still  and  move  the  screwdriver  end  into  the  coil,  and  the  needle  is 
moved  in  the  same  direction  as  when  the  coil  was  slipped  over  the 
same  end.  In  this  last  experiment  the  galvanometer  should  be  at 
least  3  ft.  away,  so  the  needle  will  not  be  affected  magnetically  by 
the  motion  of  the  screwdriver. 

1 1 1 1.     How  can  one  tell  the  direction  of  the  induced  current? 

By  common  consent  it  is  agreed  to  consider  the  lines  of  force  as 
coming  out  from  the  north-seeking  pole  of  a  magnet  and  as  going 
into 'the  south  pole.  A  convenient  rule  of  thumb  for  determining 
the  direction  of  induced  E.M.F.  or  current  is  to  hold  the  thumb  and 


FIG.  1111.— RULE  OF  THUMB. 


first  and  second  fingers  of  the  right  hand  at  right  angles  to  one 
another,  as  suggested  in  the  figure.     If  the  forefinger  points  in  the 


DYNAMOS. 


261 


direction  of  the  lines  of  force,  the  thumb  in  the  direction  of  the  mo- 
tion, the  central  finger  will  point  in  the  direction  of  the  induced 
current. 

1112.  How  long  does  an  induced  current  last? 

Only  so  long  as  the  motion  continues.  The  E.M.F.  that  causes  the 
current  is  produced  by  the  crossing  or  cutting  of  the  lines  of  force, 
and  is  proportional  to  the  rate  of  cutting. 

1113.  What  determines  the  voltage  or  pressure  of  an  induced 
current? 

The  voltage  equals  the  rate  of  cutting  lines  of  force.  It  equals 
the  product  of  the  number  of  wires  in  the  coil,  the  intensity  of  the 
magnetic  field  and  the  velocity  of  the  motion.  The  E.M.F.  is  I  volt 
when  the  product  is  100,000,000  lines  cut  per  second. 

1114.  Is  the  current  from  a  dynamo  caused  in  any  such  way  as 
described  in  No.  mo? 

The  principle  is  the  same,  although  it  is  developed  in  a  different 
way.  Instead  of  moving  a  coil  back  and  forth,  it  is  rotated  between 
the  poles  of  a  magnet,  as  indicated  in  the  figure.  If  the  coil  is  rotated 
in  the  direction  of  the  hands  of  a  clock,  the  E.M.F.  in  the  wire  at  the 


FIGS.  1114A  AND  1114B.-ELEMENTARY  DYNAMOS. 

left  tends  to  send  current  toward  the  handle,  while  that  in  the  other 
wire  is  in  the  opposite  direction.  When  both  wires  are  connected 
so  as  to  form  a  closed  circuit,  current  will  flow  around  the  circuit, 
as  indicated  by  the  arrows.  After  the  loop  has  passed  a  position 
midway  between  the  two  poles,  the  wires  begin  to  cross  the  magnetic 
lines  in  the  opposite  direction  and  the  E.M.F.  is  reversed.  The  cur- 
rent in  the  loop,  therefore,  changes  direction  also.  If  the  two  wires 


262  ELECTRICAL  CATECHISM. 

are  connected  only  at  one  end,  and  the  other  ends  are  brought  out 
and  connected  with  an  outside  circuit,  current  will  flow  in  it  also. 
The  current  obtained  from  such  a  loop  would  not  flow  continuously 
in  one  direction,  but  would  go  first  in  one  direction  and  then  in  the 
other,  being  known  as  an  alternating  current.  By  making  the  loop  to 
consist  of  many  turns  of  wire,  the  E.M.F.  is  increased  proportionally. 
The  alternating-current  generators,  as  actually  built,  have  a  number 
of  pairs  of  poles  in  the  field  magnets  and  a  corresponding  number 
of  coils  in  the  armature  (see  Nos.  1427,  1431,  1457)  ;  the  coils  are 
generally  connected  in  series,  so  that  their  E.M.F's  are  added ;  the 
ends  of  the  series  of  coils  are  connected  to  collecting  rings,  against 
which  brushes  make  contact  and  so  conduct  the  current  to  the  outside 
circuit ;  in  some  alternators  the  field  revolves  and  the  armature  is 
stationary,  in  which  case  no  collecting  rings  or  brushes  are  needed 
for  the  armature  current  (see  Nos.  1460  and  1461).  Dynamos  for 
generating  currents  which  always  flow  in  the  same  direction,  require 
development  in  a  somewhat  different  way.  If,  instead  of  coming  out 
to  two  collecting  rings,  the  ends  of  the  loop  are  connected  with  the 
halves  of  a  split  sleeve  or  cylinder,  the  brushes  (see  No.  1302)  may 
be  arranged  so  that  one  brush  makes  contact  with  one  half  the  cylin- 
der during  half  the  revolution  and  with  the  other  half  during  the  rest 
of  the  revolution;  the  brushes,  thus  making  contact  with  alternate 
halves,  may  be  set  so  that  each  brush  changes  to  the  other  half  of  the 
cylinder  at  the  same  time  that  the  E.M.F.  in  the  rotating  loop  changes 
direction ;  the  result  is  that  the  polarity  of  the  brushes  now  remains 
constant,  and  the  current  in  the  outside  circuit  goes  always  in  the 
same  direction.  The  split  sleeve  is  an  elementary  commutator. 
Current  from  such  a  source  would  be  pulsating ;  in  order  to  make  it 
more  uniform,  the  actual  armature  has  a  number  of  coils  spaced  at 
equal  angles  and  connected  in  series  so  as  to  make  a  complete  path 
around  the  armature;  between  consecutive  coils,  connections  are 
made  with  adjacent  bars  of  the  commutator,  which  has  as  many  sec- 
tions as  there  are  coils.  The  direction  of  the  induced  E.M.F's  and 
the  action  of  the  commutator  in  rectifying  the  current,  may  be  traced 
in  Fig.  11146,  which  shows  a  diagram  of  an  armature  with  eight 
coils. 

1115.     What  is  the  "field"  of  a  dynamo? 

This  word  is  used  in  three  different  senses:  (A)  It  means  the 
magnetism  or  the  magnetic  force  in  the  space  in  which  the  wires 
move ;  in  other  words,  the  magnetic  field  in  which  the  armature  re- 
volves. (B)  The  word  sometimes  refers  to  the  iron  through  which 
the  magnetic  lines  pass  from  one  pole  to  the  other.  In  many  cases 


DYNAMOS. 


263 


this  also  is  the  frame  of  the  machine.  (C)  Sometimes  men  use  the 
word,  meaning  the  "field  coil,"  the  coil  of  wire  which  is  placed 
around  part  of  the  iron  frame,  and  which  becomes  the  source  of 
magnetic  force  when  it  carries  current.  The  first  is  the  correct  use. 

1116.  What  is  the  armature? 

The  armature  is  the  part  of  the  machine  in  which  the  E.M.F.  is  pro- 
duced, or  in  which  the  current  is  generated.  As  a  general  rule,  the 
armature  revolves  and  the  field  is  stationary,  but  in  a  few  alternators 
the  armature  is  stationary  and  the  magnetic  field  revolves. 

1117.  Of  what  parts  is  the  armature  composed? 

The  armature  wires  or  coils  are  the  principal  part.  These  are  at- 
tached to  the  "armature  core/'  which  generally  consists  of  a  large 


Armature   core   ready   for  the   insertion 
of     the     coil. 


Armature   core   with    part   of   the   coils 
in  place. 


Completed  armature. 


Form   wound    coil. 


FIG.   1117.— ARMATURES. 


number  of  discs  of  sheet  iron  fastened  together  upon  a  spider  or 
directly  upon  the  shaft.    Another  part  is  the  "commutator,"  to  which 


264  ELECTRICAL   CATECHISM. 

the  ends  of  the  armature  wires  are  attached  and  with  which  the 
"brushes"  make  contact  to  carry  the  current  to  the  outside  circuit. 
In  large  dynamos,  the  iron  core  does  not  extend  to  the  shaft,  but  has 
the  center  cut  away.  In  direct-current  machines  for  high  voltage, 
such  as  arc-light  dynamos  (see  Figs.  1142  and  1162)  where  it  is 
necessary  to  have  considerable  distances  between  coils  which  have  a 
wide  difference  of  potential,  the  armature  winding  usually  consists 
of  coils,  each  of  which  surrounds  only  one  side  of  the  iron  core,  as 
indicated  in  Fig.  IH4B;  such  an  armature  is  known  as  a  Gramme 
ring,  after  its  inventor.  For  circuits  of  750  volts  or  less,  armatures 
commonly  have  the  coils  fastened  on  the  surface  or  in  slots  and  so 
spaced  that  when  one  side  of  a  coil  is  under  one  magnet  pole,  the 
opposite  side  is  in  a  corresponding  position  under  the  next  pole ;  these 
coils  may  be  wound  in  jigs  (see  example  in  Fig.  1304),  or  they  may 
be  wound  directly  on  the  armature  core,  as  in  the  accompanying  fig- 


FIG.  1117B.— A  DRUM  ARMATURE. 

ures;  such  armatures  are  called  drum  or  Siemens  armatures.  In 
any  armature,  an  important  part  is  the  insulation,  which  must  be 
carefully  selected  and  placed  so  as  to  prevent  the  possibility  of  elec- 
trical connection  between  the  windings  and  the  iron  core.  (See -also 
Nos.  1300  to  1304  and  1427  to  1470  for  motors  and  alternators.) 

1118.  Why  is  the  armature  core  made  of  thin  sheets  instead  of 
being  solid  iron? 

Because  the  iron  core  revolves  with  the  armature  wires  in  the 
magnetic  field,  and  an  E.M.F.  is  induced  in  the  iron  as  in  the  wires. 
This  E.M.F.  would  cause  currents  in  the  iron  which  would  absorb 
power,  but  which  would  not  be  useful.  Such  currents  are  called 
"eddy  currents,"  or  "Foucault  currents,"  after  their  discoverer.  By 
laminating  the  core — that  is,  by  building  it  up  of  sheets — the  paths  of 
these  currents  are  broken  up  and  they  are  prevented. 

1119.  Why  is  an  iron  core  used  for  supporting  the  armature 
wires?     Would  not  zvood  or  some  other  insulating  material  do  as 
well? 

Iron  is  used  because  it  offers  a  good  path  for  the  magnetic  field. 
The  entire  magnetic  circuit  of  the  dynamo  or  motor  is  made  of  soft 
iron  so  far  as  possible,  because  iron  makes  an  easier  path  than  air. 
By  having  the  armature  core  made  of  iron,  the  total  amount  of  mag- 


DYNAMOS, 


265 


hetism  is  greater,  and  a  much  larger  part  of  the  total  magnetism 
passes  through  the  armature.  All  of  the  magnetism  that  does  not 
pass  through  the  armature  is  lost.  (See  Nos.  745  to  758).  The 


FIG.  1119A.—  ARMATURE  DISCS. 


FIG.  1119B.— 5000-HP  ARMATURE. 


ELECTRICAL   CATECHISM. 


armature  core  discs  are  sometimes  smooth,  and  more  often  have 
notches  or  teeth.  The  teeth  improve  the  magnetic  circuit,  and  also 
support  the  armature  windings.  The  cuts  show  a  number  of  dif- 
ferent designs  of  armature  discs.  The  second  figure  shows  a  part  of 
the  armature  of  a  5ooo-hp  generator  at  Niagara  Falls,  one  of  the 
largest  generators  in  the  world. 

1120.     What  is  a  bipolar  machine? 

A  bipolar  machine  is  one  whose  field  magnet  has  only  two  poles, 
one  being  a  "north"  pole  and  the  other,  of  course,  a  "south"  pole. 


FIG.  1120.— EDISON  DYNAMO. 

The  familiar  Edison  dynamo  is  an  excellent  example  of  such  a 
machine. 

II 21.     What  is  a  multip o lar  machine ? 

A  multipolar   (abbreviated  to  "  m.p."  or  "  MP ")   machine  has 


FIG.  1L2L— MULTIPOLAR  DIRECT-CURRENT  DYNAMO. 

some  multiple  of  two  poles.    The  number  of  poles  increases  with  the 
size  of  machine,  varying  from  4  in  small  belted  machines  to  as  high 


DYNAMOS. 


267 


as  24  in  i6oo-kw.  generators  directly  connected  to  steam  engines. 
Alternators  generally  have  more  poles  than  direct-current  machines 
of  equal  output.  One  company  makes  alternators  with  6  poles  for 
30  kw.  size,  56  poles  for  3500  kw.  size ;  one  machine  has  92  poles. 
Generally  speaking,  as  the  rated  output  of  the  machine  increases,  the 
number  of  revolutions  per  minute  decreases  and  the  number  of  poles 
increases,  the  peripheral  speed  being  nearly  constant. 

1 1 22.  Does  a  dynamo  or  motor  always  have  an  even  number  of 
poles? 

There  are  as  many  "  north  "  poles  as  there  are  "  south."  The  poles 
are  sometimes  divided,  but  the  "  north  "  and  "  south  "  are  equal. 

1123.  What  is  meant  by  a  north  pole  of  a  dynamo  or  motor? 

It  is  the  pole  which  attracts  the  south-seeking  pole  of  a  compass 
needle.  The  north  pole  of  a  bipolar  machine  is  the  one  which  would 
point  toward  the  north  if  the  machine  was  freely  suspended  or  sup- 
ported on  bearings,  so  as  to  be  free  to  rotate  in  any  direction  like  a 
compass  needle. 

1124.  What  is  the  source  of  the  magnet  Held  in  dynamos? 
Some  small  machines  use  permanent  magnets  made  of  hardened 

steel.  The  machines  used  for  furnishing  light  or  power  have  electro- 
magnets, the  current  in  the  field  coils  being  taken  from  another  ma- 
chine, or  from  the  same  machine. 

1125.  What  are  magneto  machines? 

A  magneto  is  a  small  dynamo  whose  field  is  furnished  by  a  per- 


FIG.  1125.— MAGNETO. 


268 


ELECTRICAL   CATECHISM. 


manent  magnet.  A  common  form  used  for  ringing  telephone  call 
bells,  and  for  some  kinds  of  testing,  is  illustrated  in  the  figure.  The 
steel  magnets  which  give  the  field  are  seen  in  the  lower  part  of  the 
case.  The  crank  at  the  side  of  the  box  is  connected  with  a  toothed 
wheel  inside,  which  gears  into  a  smaller  wheel  on  the  end  of  the 
armature,  so  that  the  armature  makes  several  revolutions  for  one  of 
the  crank.  The  armature  itself  consists  of  a  coil  of  wire  wound 
lengthwise  around  an  iron  core,  as  shown  in  the  lower  figure.  This 
is  seen  to  be  a  development  of  the  simple  loop  shown  with  No.  1 1 14. 
A  good  magneto  will  develop  50  volts  to  100  volts  when  the  crank 
is  turned  rapidly.  It  delivers  an  alternating  current. 

1126.     Hoiv  is  a  magneto  used  for  testing? 

The  generator  and  ringer  are  connected  in  series  with  the  circuit 
to  be  tested.     When  the  handle  is  turned,  the  bell  will  ring  if  the 


L 


AH £*.  fifC. 


BELL  MAGNETO 

FIG.  1126.— MAGNETO  TESTING  BELL. 

resistance  of  circuit  is  not  too  high,  say  100,000  ohms,  the  loudness 
of  the  sound  depending  on  speed  and  resistance. 

1127.  How  can  a  magneto  be  used  to  detect  an  open  circuit? 

The  bell  will  not  ring  if  the  circuit  is  open,  unless  it  has  consider- 
able electrostatic  capacity.  The  magneto  should  be  tested  to  see  if 
it  rings  when  short-circuited  on  itself. 

1128.  How  can  a  magneto  be  used  to  detect  a  "ground"? 
Connect  one  terminal  of  the  magnet  to  the  circuit  to  be  tested,  and 

connect  -the  other  terminal  to  a  water  pipe  or  other  good  ground  con- 
nection. If  the  line  has  electrical  connection  with  the  ground  at  any 
point,  the  bell  will  ring,  its  current  passing  through  the  earth  be- 
tween the  two  ground  connections. 

1129.  How  can  a  magneto  be  used  to  locate  an  open  circuit? 
First  see  that  the  line  to  be  tested  is  free  from  ground  connections, 

as  in  all  similar  tests.  Then  connect  both  ends  of  the  line  with  the 
ground.  Go  out  to  some  convenient  point  and  open  the  line;  con- 
nect one  terminal  of  the  magneto  with  the  ground  and  connect  the 
other,  first  with  one  side  of  the  circuit  and  then  with  the  other.  If 


DYNAMOS.  269 

the  bell  rings  on  either  side  it  indicates  that  that  side  of  circuit  is  con- 
tinuous to  the  grounded  terminal.  If  the  bell  will  not  ring  on  one 
side,  it  indicates  that  the  break  is  on  that  side.  The  next  step  is  to 
close  the  circuit  again  at  the  point  tested  and  go  to  another  point  in 
the  direction  indicated  and  test  again.  By  this  process  it  is  usually 
not  difficult  to  locate  a  break. 

1130.  Is  the  magneto  always  reliable? 

No.  The  field  magnets  get  weak  with  age  and  the  pivots  of  the 
bell  hammer  get  out  of  order,  so  that  an  old  magneto  will  not  ring 
through  so  high  a  resistance  as  when  new.  Also  a  magneto  will 
sometimes  give  false  indications,  by  ringing  when  the  circuit  is  open. 

1131.  How  can  a  magneto  ring  when  the  circuit  is  open? 

It  is  because  the  magneto  gives  an  alternating  E.M.F.  Should  a 
direct  E.M.F.  be  applied  to  an  open  circuit,  only  a  transient  charging 
current  would  flow,  which  might  or  might  not  ring  the  bell. 

1132.  How  can  a  line  have  capacity? 

The  two  conductors  of  a  circuit  may  be  considered  as  the  two 
plates  or  coatings  of  a  condenser  (see  Nos.  in  to  117).  These  are 
charged  and  discharged  twice  each  revolution  of  the  magneto  arma- 
ture (see  Nos.  1114  and  1401),  current  going  first  in  one  direction 
and  then  in  the  other,  the  strength  of  current  varying  with  the 
E.M.F.  of  the  magneto  and  with  the  capacity  of  the  circuit. 

1133.  Do  electric  circuits  generally  have  capacity? 

Every  circuit  has  more  or  less  electrostatic  capacity.  On  short 
aerial  lines  it  is  rarely  important.  Telephone  cables  are  made  with 
great  care  to  reduce  capacity  to  a  minimum.  On  long  distance  lines, 
capacity  becomes  important.  (See  Nos.  115  and  1135.) 

1134.  What  determines  the  amount  of  capacity  in  a  circuit? 

The  capacity  depends  upon  the  size  of  the  two  surfaces,  their  dis- 
tance apart,  and  the  nature  of  the  substance  between  them.  It  should 
be  remembered  that  there  is  capacity  not  only  between  the  outgoing 
and  incoming  wires  of  a  circuit,  but  also  between  each  wire  and  the 
earth.  In  the  case  of  underground  conductors,  the  capacity  between 
two  wires  and  between  each  one  and  the  earth  may  be  considerable. 

1135.  Is  the  capacity  of  a  circuit  of  any  special  importance? 
Capacity  is  generally  unimportant  on  short  lines  for  direct  current 

at  low  potential  difference;  its  effect  is  magnified  by  high  voltages 
and  high  frequencies  such  as  are  common  with  lightning.  Thus, 


270 


ELECTRICAL  CATECHISM. 


a  condenser  with  plates  the  size  of  a  silver  dollar  may  vitiate  the 
working  of  a  jump-spark  coil  on  a  gas  engine.  With  the  ordinary 
alternating  current  frequencies  of  2$  or  60  cycles  per  second,  the 
electrostatic  capacity  of  long  distance  transmission  lines  limits  the 
smallest  amount  of  power  that  can  be  handled.  For  example,  electro- 
statically charging  the  65-mile  line  between  Canyon  Ferry  and  Butte, 
Mont.,  requires  a  current  of  about  14  amperes  at  66,000  volts, 
involving  over  2000  apparent  horsepower  independent  of  power 
delivered. 

1136.  Is  not  the  word  "capacity"  used  with  several  different 
meanings? 

The  word  sometimes  means  electrostatic  capacity,  as  just  dis- 
cussed; it  sometimes  refers  to  the  amount  of  current  a  conductor 
will  carry  without  unsafe  heating,  and  it  is  also  used  with  reference 
to  the  safe  or  greatest  output  of  a  machine.  It  is  generally  not  diffi- 
cult to  tell  which  meaning  is  intended. 


FIG.  1137.— ALTERNATOR  WITH  EXCITER. 


1137.     What  is  a  separately  excited  dynamo  f 
It  is  one  in  which  the  current  in  the  field  magnet  coils  comes  from 
another  source  than  the  machine  itself. 


DYNAMOS. 


271 


1138.  What  kind  of  dynamos  are  separately  excited f 

It  is  common  to  use  a  small  auxiliary  dynamo  called  an  exciter  to 
furnish  current  for  the  field  coils  of  alternators.  Sometimes  also 
large  continuous  current  dynamos  are  separately  excited,  especially 
when  used  on  a  grounded  circuit,  as  for  electric  railway  work. 

1139.  What  classes  of  dynamos  excite  their  own  fields? 

These  are  sometimes  called  "self-excited"  machines,  and  may  have 
the  field  coils  connected  "in  series,"  or  "in  shunt,"  or  they  may  be 
"compound  wound." 

1140.  What  is  a  series  dynamo? 

A  series  dynamo  is  one  in  which  the  whole  current  from  the  ar- 
mature passes  through  the  field  magnet  coils,  as  suggested  in  the 


O 


FIG.  1140.-SERIES  DYNAMO. 

figure.  The  field  coils  consist  of  comparatively  coarse  wire,  usually 
about  twice  the  size  of  the  wire  on  the  armature. 

1 141.  For  what  purposes  are  series  dynamos  used? 
Principally  for  arc  lighting. 

1 142.  What  sort  of  current  is  obtained  from  series  dynamos? 
Continuous,  constant  current ;  that  is,  the  current  always  goes  in 

the  same  direction,  and  is  of  nearly  constant  strength.  The  current 
from  a  series  dynamo  actually  does  vary  to  some  extent,  unless  the 
machine  has  a  good  regulator,  being  greater  as  the  number  of  lamps 
or  other  resistance  in  the  circuit  becomes  less. 

1143.  How  much  current  is  given  by  series  dynamos? 
Dynamos   supplying   "  full   arcs "   of  2000  nominal   c-p.   of  the 

"  open  "  variety,  are  generally  regulated  for  9.6  amperes ;  when  the 
arcs  are  enclosed,  the  current  is  set  for  6.6  amperes.  For  "  half 
arcs  "of  1200  nominal  c-p.,  the  current  is  adjusted  for  6.8  amperes 
when  the  arcs  are  open,  or  for  5  amperes  when  they  are  enclosed. 
(See  Nos.  484  to  493.) 


ELECTRICAL  CATECHISM. 


FIG.   1142.— BIPOLAR  ARC   LIGHT   DYNAMO. 

1 144.  Why  are  arc  lamps  rated  at  so  many  nominal  candle-power? 
Because  the  light  is  actually  less  than  was  once  supposed.     An 

arc  light  that  was  formerly  supposed  to  be  equal  to  2000  candles  is 
now  known  to  be  equal  to  about  1200  candles  in  the  direction  of 
greatest  intensity ;  that  is,  at  about  half-way  between  the  horizontal 
and  vertical.  The  average  strength  of  light  in  all  directions  is  about 
800  candles.  Much  trouble  has  been  caused  from  the  discrepancy 
between  the  actual  and  nominal  candle-power,  and  it  is  more  common 
to  specify  nominal  c-p.,  or  better,  watts  taken  by  lamps. 

1145.  What  is  the  voltage  of  series  dynamos f 

In  each  dynamo  this  varies  with  the  number  of  lamps,  being  higher 
as  more  lamps  are  in  circuit.  Each  "open"  arc  lamp  requires  about 
50  volts.  "Enclosed"  arc  lamps  usually  require  about  80  volts  each. 
Arc-light  dynamos  are  made  in  different  sizes  to  operate  from  10  to 
220  lights. 

1146.  What  is  a  shunt  dynamo  f 

A  shunt  dynamo  has  the  field  magnets  excited  by  means  of  a  coil 
of  many  turns  of  comparatively  fine  wire.  The  field  coil  is  connected 
to  both  terminals  of  the  armature,  so  that  a  small  part  of  the  arma- 
ture current  goes  through  the  field  coil  and  the  balance  of  the  cur- 
rent goes  through  the  outside  circuit,  as  suggested  in  the  figure.  The 
field  coil  is  thus  a  shunt  or  side  circuit  to  the  main  circuit. 

1 147.  What  sort  of  current  is  obtained  from  shunt  dynamos? 

The  voltage  or  pressure  is  approximately  constant,  but  the  cur- 
rent varies  according  to  the  number  of  lamps  lighted.  The  voltage 
becomes  less  as  the  current  increases,  unless  the  machine  is  regulated. 


DYNAMOS. 


FIG.  1146.— SHUNT  WOUND  DYNAMO. 

1 148.  For  what  purposes  are  shunt  dynamos  used? 

For  furnishing  current  for  incandescent  lamps,  motors,  heaters 
and  to  an  increasing  extent,  for  arc  lamps. 

1149.  Hozv  is  a  shunt  dynamo  regulated? 

By  means  of  an  adjustable  resistance  in  a  resistance  box  or  rheo- 
stat, connected  in  series  with  the  field  coil.  By  cutting  out  part  of 
this  resistance,  more  current  goes  through  the  field  coil.  This 
strengthens  the  magnet,  and  so  increases  the  voltage  of  the  dynamo. 
In  a  similar  way  the  voltage  is  made  lower  by  putting  more  resistance 
in  the  field  circuit.  The  rheostat  is  generally  adjusted  by  hand. 


FIG.  1150A.— COMPOUND  WOUND  DYNAMO. 


274 


ELECTRICAL   CATECHISM. 


H5'o.     What  are  compound  dynamos? 

Compound  dynamos  are  shunt  dynamos  having  additional  series 
field  coils.    As  the  current  increases  and  the  voltage  would  decrease 


120 
110 
100 
90 

S  70 

>  60 
50 
40 
80 
20 
10 
0 


COMPOUND 


0 


120      ICO     200     240     280 
Amperes 


FIG.  1150B.-CHARACTERISTICS  OF  COMPOUND  DYNAMO. 

with  a  simple  shunt  machine,  the  current  through  the  series  coils 
strengthens  the  magnetic  field,  and  so  raises  the  voltage,  or  at  least 
keeps  it  from  falling.  (See  No.  1244.) 

1151.  What  is  an  over-compounded  dynamo  f 

A  machine  is  said  to  be  over-compounded  when  the  series  coils  are 
so  strong  that  the  voltage  becomes  higher  as  the  current  increases. 
(See  No.  1245.) 

1152.  Why  are  machines  over-compounded? 

In  order  to  keep  the  voltage  constant  at  some  point  at  a  distance 
from  the  dynamo.  As  the  current  on  a  line  increases,  the  "drop,"  or 
volts  lost  on  a  line,  increases  proportionally.  Unless  the  voltage  at 
the  dynamo  increases  accordingly,  the  lamps  have  lower  voltage 
and  become  dull  and  unsatisfactory. 

1153.  What  is  a  (< teaser  coil"? 

In  early  days  a  compound  coil  used  on  electroplating  machine  was 
called  the  teaser  coil.  In  the  monocyclic  alternating-current  system, 
the  auxiliary  armature  winding  is  sometimes  called  the  teaser  wind- 
ing, the  line  wire  leading  from  it  the  teaser  wire,  and  the  voltage  be- 
tween the  latter  and  either  of  the  main  line  wires,  the  teaser  voltage. 

1154.  What  iixes  the  current  given  by  a  dynamo? 

The  output  of  a  dynamo  is  governed  by  the  simple  and  well-known 
principle  called  "Ohm's  law" — the  current  equals  the  voltage  divided 
by  the  resistance.  If  a  dynamo  gives  a  certain  voltage,  the  current 
will  depend  upon  the  resistance  in  the  circuit,  being  greater  as  the 
resistance  becomes  less.  (See  Nos.  315  to  334.) 


DYNAMOS.  275 

1155.  Does  the  resistance   of  an  arc-lighting  circuit  increase 
when  more  lamps  are  turned  on? 

Yes.  All  the  lamps  are  generally  in  series,  so  that  the  same  current 
passes  through  all,  one  after  the  other.  From  40  volts  to  60  volts 
must  be  supplied  by  the  dynamo  for  each  arc  lamp  in  a  series  circuit. 

1156.  How  is  the  current  kept  constant  when  more  lamps  are 
connected  into  a  series  circuit? 

If  the  current  is  to  remain  constant  the  voltage  must  increase  as 
the  resistance  increases.  Each  lamp  added  in  a  series  circuit  in- 
creases the  resistance  of  the  circuit,  and-  so  increases  the  voltage 
necessary  at  the  dynamo. 

1157.  Does  the  resistance  of  an  incandescent  lighting  circuit  in- 
crease when  more  lamps  are  connected? 

No.  Incandescent  lamps  are  generally  connected  in  multiple  be- 
tween the  mains,  somewhat  like  the  rounds  of  a  ladder.  Each  lamp 
gets  its  own  current  almost  independently  of  the  others,  as  each  gas 
jet  or  water  faucet  is  independent.  The  larger  the  number  of  lamps 
connected  in  multiple  the  greater  is 'the  current ;  just  as  the  more  gas 
jets  are  open,  the  more  gas  is  burned.  If  the  lamps  are  all  of  the 
same  candle-power,  their  resistances  are  about  equal,  and  the  com- 
bined resistance  of  all  the  lamps  is  practically  the  resistance  of  one 
divided  by  the  number  of  lamps. 

1158.  Is  there  any  general  principle  by  which  all  dynamos  are 
regulated  to  give  constant  current  or  constant  voltage? 

All  dynamos,  whether  for  arc  or  incandescent  lighting,  and 
whether  giving  continuous  or  alternating  currents,  regulate  by  ad- 
justing the  voltage.  Constant-current  dynamos  must  have  the  voltage 
increase  or  decrease  accordingly  as  the  resistance  of  the  circuit 
changes.  Constant-potential  dynamos  generally  require  some  regu- 
lation to  keep  the  voltage  constant,  because  there  is  a  tendency  foi 
the  voltage  to  fall  as  the  current  increases. 

1159.  What  governs  the  voltage  of  a  dynamo? 

The  voltage  equals  the  rate  of  cutting  magnetic  lines  of  force ; 
that  is,  the  product  of  the  number  of  r.  p.  m.,  the  number  of  con- 
ductors on  the  armature  and  the  number  of  magnetic  lines  of  force 
through  the  armature  (see  Nos.  1113  and  1114).  The  voltage  may 
be  varied  by  changing  any  one  of  the  three  factors. 

1160.  Are  dynamos  ever  regulated  by  changing  the  speed  of  the 
armature? 

The  engine  governor  is  sometimes  changed  so  as  to  run  faster 
with  large  load.  This  is  not  suitable  for  alternating  current 


£76  ELECTRICAL  CATECHISM. 

generators  operating  in  multiple  or  supplying  a  motor  load,  but 
will  do  for  direct  current  machines.  A  common  case  is  in  the 
use  of  the  ordinary  magneto.  The  voltage  depends  on  the  speed  of 
the  armature,  and  the  faster  the  handle  is  turned  the  higher  the 
voltage.  By  turning  the  handle  faster,  the  bell  will  ring  through  a 
higher  resistance.  (See  Nos.  1125  and  1126).  This  method  of 
regulation  is  not  generally  suitable  for  dynamos.  Frequently  the 
voltage  is  affected  by  changes  of  speed  that  are  not  desired,  as  often 
happens  when  dynamos  are  driven  from  the  same  power  as  other 
machines  whose  load  is  changing. 

1161.  Is  it  possible  to  regulate  voltage  by  changing  the  number  of 
conductors  while  a  machine  is  running? 

A  number  of  arc  dynamos  regulate  in  practically  this  way.  Some 
use  two  brushes  on  each  side  and  spread  the  brushes,  thus  short- 
circuiting  some  of  the  sections  of  the  armature.  In  the  Thomson- 
Houston  arc  machines  the  whole  armature  is  short-circuited  six  times 
every  revolution  for  a  longer  or  shorter  period  varying  with  the 
position  of  the  brushes. 

1162.  Do  not  some  machines  regulate  the  voltage  by  shifting  the 
brushes ? 

Yes.  An  ordinary  constant-potential  dynamo,  such  as  used  for  in- 
candescent lighting,  is  affected  to  a  considerable  extent  by  the  po- 
sition of  the  brushes. 

The  Western  Electric  arc  dynamo  (Fig.  1142)  is  regulated  en- 
tirely by  shifting  the  brushes,  which  are  placed  so  as  to  touch  the 
commutator  bars  connected  with  armature  sections  directly  under 
the  pole  pieces,  so  that  some  of  the  wires  are  inducing  E.M.F  in  the 
opposite  direction  to  the  rest.  Shifting  the  position  of  the  brushes 
changes  the  number  of  wires  working  against  the  main  E.M.F.  This 
changes  the  effective  number  of  wires  on  the  armature  and  also  af- 
fects the  distortion  of  the  magnetic  field  by  the  armature  current.  This 
method  was  used  with  Standard,  Sperry  and  Edison  arc  dynamos.  The 
Fort  Wayne  (Wood)  arc  machine  regulates  by  shifting  and  spreading 
the  brushes.  The  Excelsior  and  the  new  Brush  arc  dynamos  shift 
the  brushes  and  also  change  the  strength  of  the  field  magnet. 

1163.  Are  many  dynamos  regulated  by  changing  the  number  of 
magnetic  lines  of  force  through  the  armature? 

Yes.  All  constant-potential  or  incandescent-lighting  dynamos, 
and  some  constant-current  or  arc-lighting  dynamos  regulate  by  this 
method. 


DYNAMOS. 


FIG.  1162.— BRUSH    ARC   GENERATOR    FOR    FOUR  CIRCUITS. 

1164.  What  different  ways  are  there  for  changing  the  number 
of  magnetic  lines  of  force  through  the  armature? 

Either  the  total  number  of  magnetic  lines  (the  total  amount  of 
magnetism)  may  be  changed,  or  the  useful  proportion  of  the  total 
number  may  be  changed. 

1165.  How  can  the  useful  proportion  of  the  whole  amount  of 
magnetism  be  changed? 

By  providing  another  path  beside  the  armature.  (See  Nos.  1119, 
747,  756  and  757).  Some  small  machines  have  a  movable  soft  iron 
bar  that  may  be  placed  so  as  to  deflect  more  or  less  of  the  magnetism 
and  conduct  it  from  one  pole  to  the  other  without  passing  through 
the  armature.  This  is  an  unusual  method  and  used  only  for  small 
medical  machines. 

Another  plan  once  used  in  an  English  arc  lighting  machine  was  to 


278  ELECTRICAL  CATECHISM. 

have  a  double  magnetic  circuit,  one  of  which  furnished  the  magnetic 
field,  while  the  other  shunted  more  or  less  around  the  armature.  The 
shunting  part  was  surrounded  by  coils  of  wire  through  which  an 
adjustable  current  passed.  This  plan  is  no  longer  used. 

1 1 66.  What  different  ways  are  there  for  varying  the  total  amount 
of  magnetism? 

The  number  of  lines  of  force  in  a  magnetic  circuit  is  proportional 
to  the  product  of  the  number  of  turns  of  wire  by  the  amount  of  cur- 
rent in  the  magnetizing  coil  and  is  inversely  proportional  to  the  re- 
luctance of  the  magnetic  circuit.  Any  one  of  the  three  may  be  varied. 

1167.  What  is  meant  by  reluctance? 

The  reluctance  of  a  magnetic  circuit  corresponds  with  the  resist- 
ance of  an  electric  circuit.  The  reluctance  is  proportional  to  the 
length  of  the  circuit ;  it  is  inversely  proportional  to  the  area  of  cross 
section  and  also  to  the  permeability  of  the  circuit.  (See  Nos.  724  to 

726.) 

1168.  Can  the  reluctance  of  a  dynamo  be  changed  while  the  ma- 
chine is  running? 

In  a  few  small  machines,  parts  of  the  iron  field  circuit  are  movable 
so  as  to  lengthen  or  shorten  the  "air  gap." 

1 1 69.  What  is  the  air  gap  ? 

A  name  given  to  the  part  of  the  magnetic  circuit  composed  of  air. 
Usually  the  air  gap  is  between  the  pole  pieces  and  the  iron  core  of  the 
armature.  More  or  less  air  gap  is  necessary  to  provide  room  for  the 
armature  wires  and  also  for  mechanical  clearance  between  armature 
and  pole  pieces. 

1170.  How  can  the  amount  of  current  in  the  magnetizing  coil  be 
varied? 

In  shunt-wound  machines  this  may  be  varied  by  changing  the  re- 
sistance in  the  rheostat.  (See  No.  1149).  With  series-wound  ma- 
chines (No.  1140)  an  adjustable  resistance  is  sometimes  shunted 
around  the  series  field  Coil  so  that  the  field  may  be  weakened  by  al- 
lowing more  or  less  current  to  pass  around  the  coil.  The  Brush  arc 
dynamo  regulates  in  this  way.  By  reducing  the  resistance  in  the 
shunt,  less  current  goes  through  the  field  coils  so  that  the  magnetic 
field  becomes  weaker,  and  consequently  the  E.M.F.  of  the  armature 
is  reduced. 

1171.  How  is  a  shunt  dynamo  started? 

If  the  machine  has  been  running  before  and  everything  is  in  good 
order,  simply  let  down  the  brushes  so  that  they  press  against  the  com- 


DYNAMOS.  279 

mutator,  and  bring  the  machine  up  to  speed.  If  everything  is  all 
right,  the  field  magnetism  will  "pick  up"  as  the  machine  comes  up 
to  speed,  and  in  a  short  time  the  machine  will  be  generating  full 
voltage.  The  voltage  may  be  adjusted  up  or  down  by  moving  the 
rheostat  handle. 

1172.  How  does  the  field  of  a  shunt  dynamo  pick  up? 

After  a  machine  is  stopped,  the  iron  of  the  field  magnet  retains 
more  or  less  magnetism,  being,  in  fact,  a  semi-permanent  magnet. 
This  residual  magnetism  furnishes  a  weak  field  which  causes  a  small 
E.M.F.  to  be  induced  in  the  wires  'on  the  armature  as  the  machine 
comes  up  to  speed.  If  the  armature  and  field  coils  form  part  of  a 
closed  circuit,  the  small  E.M.F.  induced  by  the  residual  magnetism 
sends  a  small  current  through  the  circuit.  This  current  strengthens 
the  magnetic  field  and  causes  a  still  higher  E.M.F.  to  be  induced. 
This  again  sends  still  more  current  through  the  field  coils,  and  the 
machine  thus  quickly  builds  up  its  magnetism  and  voltage  to  full 
strength. 

1173.  What  prevents  the  voltage  of  a  shunt  dynamo  from  in- 
creasing indefinitely? 

The  machine  is  so  designed  that  the  iron  of  the  field  magnet  be- 
comes more  or  less  saturated  by  the  time  the  voltage  has  risen  to  the 
desired  amount. 

1  174.     What  is  meant  by  the  saturation  of  iron? 

This  may  be  explained  by  the  accompanying  figure,  in  which  the 
curved  line  represents  the  relation  between  the  magnetizing  current 
and  the  resulting  magnetization.  After  the  iron  has  been  magnetized 
once  it  retains  a  certain  amount,  as  indicated  by  the  curve,  beginning 


i 


MAGNETIZING    CURRENT 

FIG.  1174.-SATU  RATION  CURVE. 

at  some  distance  above  zero.  For  small  magnetizing  forces  the  per- 
meability of  the  iron  is  small,  and  the  magnetization  increases  slowly. 
The  permeability  then  increases  and  the  magnetization  increases 
rapidly  until  the  iron  approaches  the  saturation  limit.  The  upper 
sharp  bend  in  the  curve  is  called  the  saturation  point,  since  after  the 
iron  has  been  magnetized  up  to  this  point,  it  is  difficult  to  magnetize 
it  much  more  strongly.  (See  also  No.  763.) 


280  ELECTRICAL   CATECHISM. 

1175.  How  can  one  tell  the  normal  speed  of  a  dynamo ? 
Usually  the  speed  is  stamped  on  the  name  plate  on  the  machine 

or  on  the  end  of  the  armature  shaft.  Belt-driven  machines  usually 
have  a  velocity  of  3000  ft.  per  minute  as  measured  around  the  outside 
of  the  armature,  although  some  machines  run  at  4000  ft.  or  more. 
Machines  directly  coupled  to  engines  sometimes  run  as  slowly  as 
1600  ft.  per  minute. 

1176.  How  can  one  tell  the  normal  voltage  of  a  shunt  dynamo? 
A  shunt  dynamo  usually  has  quite  a  wide  range  of  voltages,  at 

which  it  will  operate  successfully.  It  is  common  to  have  a  machine 
able  to  generate  about  50  per  cent  above  normal  voltage  when  the 
main  switch  is  open  so  that  the  machine  delivers  no  current  except 
for  its  own  fields,  when  the  resistance  is  all  cut  out  of  the  shunt  field 
rheostat  and  when  the  machine  is  running  at  full  speed.  It  is  now 
customary  to  make  shunt  dynamos  for  125,  250  and  600  volts. 

1177.  How  can  one  tell  the  full  load  current  of  a  shunt  dynamo? 
If  this  is  not  marked  upon  the  name  plate  or  armature  shaft,  one 

can  tell  either  by  running  the  machine  for  one  or  two  hours  or  by 
finding  the  size  of  the  wire  on  the  armature.  Modern  dynamos  are 
so  designed  that  the  temperature  of  the  armature  will  not  rise  more 
than  50  deg.  cent,  above  the  temperature  of  the  room  after  giving 
full-load  current  for  6  to  18  hours. 

1178.  How  can  one  determine  the  temperature  of  an  armature? 
Have  a  thermometer  that  reads  as  high  as  212  degs.  F.,  or  100 

degs.  C,  preferably  a  straight  one  without  a  wooden  or  tin  case, 
and  with  the  scale  marked  on  the  glass  tube.  Immediately  after  the 
machine  has  stopped  running  lay  the  thermometer  upon  the  arma- 
ture in  such  a  position  that  the  bulb  rests  directly  upon  the  arma  • 
ture  wires  or  on  the  iron  core.  Cover  the  thermometer  carefully  with 
waste  or  something  similar,  so  as  to  hold  the  bulb  in  position,  and  also 
to  prevent  any  currents  of  air  from  cooling  it.  In  a  few  minutes  the 
thermometer  will  indicate  the  temperature  of  the  armature.  The  dif- 
ference between  this  and  the  reading  of  a  similar  thermometer  hung 
in  the  air  a  few  feet  from  the  machine  gives  the  rise  of  temperature 
of  the  armature.  Temperature  rise  in  machine  windings  is  preferably 
determined  by  the  increase  in  resistance.  (See  No.  340.) 

1179.  Is  it  true  that  an  armature  is  cooler  while  running  than 
after  it  stops? 

It  is  true  with  armatures  that  are  not  thoroughly  ventilated,  be- 
cause   while  running  the  armature  creates  a  current  of  air  like  a 


DYNAMOS.  28] 

fan  that  carries  off  the  heat  from  the  surface,  so  that  the  surface  is 
cooler  than  the  inside  of  the  armature.  When  the  armature  stops,  the 
current  of  air  also  stops,  and  the  whole  of  the  armature  comes  to 
about  the  same  temperature.  Therefore,  the  surface  becomes  warmer 
for  some  time  after  the  armature  stops  running. 

1 1 80.  How  can  one  tell  the  allowable  current  from  the  size  of  the 
armature  wire? 

Determine  the  size  of  the  wire  and  the  total  number  of  circuits 
among-  which  the  armature  current  is  divided.  Determine  also 
whether  each  path  consists  of  a  single  wire  or  of  two  or  more  wound 
as  one.  From  a  wire  table,  find  the  number  of  "circular  mils"  in  the 
wire  used,  multiply  by  the  total  number  of  wires  in  parallel,  and 
divide  by  600.  The  result  is  about  the  number  of  amperes  the  arma- 
ture can  probably  deliver  without  excessive  heating. 

1181.  Why  divide  by  600? 

Because  experience  shows  that  600  circ.  mils  per  amp.  is  about  the 
smallest  size  of  wire  allowable.  This  figure  varies  in  different  cases 
from  325  to  650. 

1182.  How  can  one  tell  the  number  of  paths  for  the  current  in  an 
armature? 

In  a  two-pole  machine  the  current  divides  into  two  equal  parts, 
each  side  carrying  half.  In  multipolar  machines  having  an  even 
number  of  commutator  segments,  the  number  of  paths  in  the  arma- 
ture is  the  same  as  the  number  of  poles  in  the  field.  If  the  number 
of  commutator  segments  is  an  odd  number,  such  as  95  or  101,  the 
armature  has  a  two-path  winding.  Alternator  armatures  have  either 
a  single  or  double  path,  the  double  path  being  indicated  where  the 
leads  to  the  collecting  rings  come  from  opposite  sides  of  the  arma- 
ture. 

1183.  In  starting  a  shunt  dynamo,  should  the  main  line  switch  be 
closed  before  the  machine  is  up  to  voltage  or  after? 

If  the  machine  is  working  on  the  same  circuits  with  other  ma- 
chines, or  with  a  storage  battery,  it  is,  of  course,  necessary  to  make 
the  voltage  of  the  machine  equal  to  that  on  the  line  before  connecting 
it  in.  If  the  machine  works  alone,  the  switch  may  be  closed  either 
before  or  after  the  voltage  comes  up.  The  load  is  thrown  on  sud- 
denly if  the  switch  is  closed  after  the  machine  is  up,  thus  causing  a 
strain  on  the  belt,  and  possibly  drawing  water  over  into  the  engine 
cylinder.  On  the  other  hand,  if  the  switch  is  closed  before  the 
voltage  of  the  machine  has  come  up,  the  load  is  picked  up  gradually, 
but  the  machine  may  be  slow  or  may  even  refuse  to  pick  up  at  all. 


282 


ELECTRICAL  CATECHISM, 


1 184.  Why  does  a  shunt  machine  pick  up  more  slowly  if  the  main 
switch  is  closed  first? 

Because  the  resistance  of  the  main  line  is  so  much  less  than 
that  of  the  field  that  the  small  initial  E.M.F.  due  to  the  residual  mag- 
netism, causes  a  much  larger  current  in  the  armature  than  in  the 
shunt  field.  If  this  is  too  large,  the  cross  and  back  magnetizing  force 
of  the  armature  weakens  the  field  more  than  the  initial  field  current 
strengthens  it,  and  so  the  machine  can  not  build  up. 

1185.  //  a  shunt  dynamo  will  not  pick  up,  what  is  likely  to  be 
the  trouble? 

The  speed  may  be  too  low;  the  resistance  of  the  external  circuit 
may  be  too  small;  the  brushes  may  not  be  in  the  proper  position; 
some  of  the  electrical  connections  in  the  dynamo  may  be  loose, 
broken  or  improperly  made ;  the  field  may  have  lost  its  residual  mag- 
netism. 

1 186.  How  can  one  tell  if  the  speed  is  too  low? 

The  proper  speed  is  usually  stamped  on  the  name  plate  of  the  ma- 
chine, or  on  one  end  of  the  armature  shaft.  If  not  marked  on  the 


FIG.  1186A.— REVOLUTION  COUNTER. 


FIG.  1186B.— TACHOMETER. 


machine,  it  may  be  found  in  the  catalogue  of  the  manufacturer. 
Otherwise  it  may  be  estimated  as  indicated  in  No.  1175.  The  actual 
speed  of  the  armature  may  be  measured  by  a  revolution  counter  and 
.watch,  or  by  a  tachometer,  the  sharp  end  of  the  spindle  of  the  instru- 


DYNAMOS.  283 

ment  being  held  against  the  end  of  the  armature  shaft  so  that  the 
two  revolve  at  the  same  speed.  Sometimes  a  convenient  way  is  to 
count  the  engine  speed  by  one's  fingers,  and  then  get  the  dynamo 
speed  by  multiplying  the  engine  speed  by  the  ratio  of  diameters  of 
engine  and  dynamo  pulleys. 

1187.  How  can  the  speed  of  an  engine  be  counted  if  one  has  no 
speed  indicator? 

If  the  engine  does  not  run  faster  than  about  100  r.  p.  m.,  one  can 
count  the  number  of  strokes  in  a  minute  by  keeping  one  eye  on  a 
watch,  while  the  other  watches  the  cross-head  or  piston  rod.  Some- 
times one  can  stand  so  that  the  cross-head  or  valve  stem  will  touch 
the  hand  at  each  stroke.  When  an  engine  runs  too  fast  for  one  to 
count,  he  can  get  the  speed  by  the  use  of  the  fingers  of  both  hands, 
as  follows :  Keep  a  watch  where  its  second  hand  can  be  seen  at  the 
same  time  as  the  valve  rod  or  piston  rod ;  by  a  little  practice  one  can 
get  the  fingers  of  the  right  hand  moving  so  that  one  finger  comes 
down  for  each  stroke  of  the  engine ;  one  finger  of  the  left  hand  is 
brought  down  for  each  five  or  ten  of  the  first ;  in  this  way  each  finger 
of  the  left  hand  represents  five  revolutions,  and  going  over  the  whole 
left  hand  represents  twenty-five  revolutions;  by  bending  one  finger 
each  time  the  left  hand  is  covered,  one  may  keep  track  of  twenty- 
fives,  and  so  can  keep  count  of  125  revolutions.  Some  prefer  to 
keep  tally  on  the  vest  buttons.  By  this  method  it  may  be  necessary 
to  count  the  revolutions  in  a  half  or  quarter  minute  and  then  multiply 
by  two  or  four.  By  practice  one  can  determine  the  speed  of  an  engine 
or  shaft  running  as  high  as  300  r.  p.  m. 

1 1 88.  How  can  one  tell  whether  the  brushes  are  in  the  proper 
positions? 

If  there  is  little  or  no  sparking  when  the  machine  is  working,  it 
is  safe  to  conclude  the  brushes  are  set  properly.  On  bi-polar 
machines  the  brushes  should  bear  upon  the  commutator  at  points 
diametrically  opposite;  in  general,  the  brushes  should  be  so  spaced 
as  to  divide,  the  commutator  into  as  many  equal  parts  as  there  are 
poles  in  the  field  magnet.  Most  shunt  dynamos,  except  Edison  ma- 
chines, should  have  the  brushes  touch  the  commutator  at  points  op- 
posite the  space  between  the  pole  pieces,  as  indicated  in  Fig.  11503. 
Care  should  be  taken  that  the  brushes  make  good  electrical  contact 
with  the  commutator.  Sometimes  the  armature  has  so  much  end  play 
that  the  brushes  make  poor  contact,  a  temporary  remedy  for  this  be- 
ing to  hold  a  stick  against  one  end  of  the  armature  so  as  to  stop  end 
play  until  the  machine  has  picked  up.  Care  should  be  taken  also 


284  ELECTRICAL  CATECHISM. 

that  the  springs  press  the  brushes  firmly,  but  flexibly,  against  the 
commutator. 


FIG.  1189.— MULTIPOLAR   DYNAMO    (DRIVEN   BY   GAS   ENGINE). 

1189.  Why  are  the  brushes  of  a  four-pole  dynamo  placed  90  degs. 
apart,  instead  of  180? 

Because  of  the  arrangement  of  the  winding.  There  are  two  dis- 
tinct generating  circuits  in  a  four-pole  machine,  one  corresponding 
to  one  pair  of  adjacent  poles,  and  the  other  to  the  other  pair;  as  in 
the  case  of  a  bipolar  machine,  the  position  of  the  brushes  for  each 
circuit  is  between  the  poles,  which  brings  the  brushes  90  degs.  apart. 
Instead,  however,  of  using  four  brushes,  the  two  circuits  are  some- 
times connected  in  parallel,  thus  reducing  the  brushes  to  two,  al- 
though it  is  common  to  use  four  brushes  to  reduce  the  necessary 
length  of  commutator. 

1190.  How  are  the  brushes  of  a  dynamo  located? 

On  some  machines,  the  brushes  are  placed  so  as  to  bear  on  com- 
mutator bars  almost  opposite  the  middle  of  the  polepieces.  On 
others  they  are  almost  opposite  a  point  midway  between  the  poles. 
The  position  depends  on  the  relative  position  of  the  commutator  con- 
nections with  reference  to  the  windings,  and  is  governed  by  con- 
venience of  connections  or  of  access  to  the  brushes  (see  No.  1120). 


DYNAMOS. 


285 


1191.  Should  the  brushes  of  a  dynamo  be  in  the  same  position 
with  a  large  load  as  ivith  a  small  load? 

Some  machines  are  so  designed  that  no  shifting  of  the  brushes  is 
necessary.  Usually  the  brushes  must  be  moved  forward,  in  the 
direction  the  armature  rotates,  as  the  load  becomes  larger.  This 
is  to  reduce  the  sparking. 

1 192.  Of  what  materials  are  dynamo  brushes  made? 

They  are  made  of  copper  sheets,  copper  wire  gauze,  combinations 
of  copper  and  high-resistance  metal,  and  are  often  made  of  carbon. 


FIG.  1192A.— BRUSH-HOLDERS  WITH  CARBON  BRUSHES. 

Strip  copper  was  used  almost  exclusively  for  many  years.  Carbon 
is  generally  used  with  large  dynamos,  especially  when  delivering  cur- 
rent at  more  than  200  volts.  Copper  has  the  highest  conductivity 
and  carries  the  current  to  and  from  the  armature  with  the  least  loss. 
When  an  armature  is  delivering  a  considerable  current  there  is  apt 
to  be  more  or  less  sparking  at  the  brushes  on  account  of  armature 
reactions.  The  brush  should  have  high  conductivity  lengthwise,  but 
low  conductivity  crosswise,  the  latter  being  found  to  reduce  the 
sparking.  If  the  total  voltage  is  high,  a  little  loss  at  the  brushes 
does  not  make  much  difference,  and  high-resistance  brushes  of  car- 
bon are  suitable  for  such  machines.  For  low-voltage  machines,  such 


286  ELECTRICAL  CATECHISM. 

as  generators  for  125  volts  or  less,  the  loss  in  the  brushes  may  be- 
come a  considerable  percentage  of  the  whole,  so  that  other  brushes 
are  preferable.  The  tendency  for  sparking  is  reduced  considerably 
by  putting  sheets  of  poor  conductor,  such  as  thin  paper  between  the 
copper  sheets.  In  the  Wirt  brushes,  the  central  part  is  of  copper, 
while  the  outer  sheets  are  of  high-resistance  metal,  which  cuts  down 
the  sparking. 


FIG.  1193B.— METALLIC   BRUSHES. 

1193.  What  causes  sparking  at  the  brushes? 

Sparking  occurs  whenever  there  is  a  difference  of  potential  be- 
tween the  brush  and  the  part  of  the  commutator  that  is  passing  out 
from  under  the  brush.  It  is  necessary  that  at  times  two  commutator 
bars  come  under  the  brush,  and  so  the  armature  coil  between  them  is 
short-circuited  for  an  instant.  If  the  machine  is  properly  designed 
and  made,  and  if  the  brushes  are  properly  placed,  there  should  be  no 
difference  of  potential  between  the  commutator  bar  just  coming 
under  a  brush  and  the  bar  just  leaving  it.  This  condition  is  not 
always  met,  and  there  is  apt  to  be  more  or  less  voltage  between  the 
two  bars,  due  to  the  magnetic  action  of  the  current  in  the  armature. 
Such  voltage  will  send  a  current  across  the  face  of  the  brush  in- 
dependently of  the  main  current,  and  the  reason  for  using  the  high- 
resistance  carbon  or  other  brush  is  to  reduce  this  cross  current,  and 
therefore  reduce  the  cutting  of  the  commutator  and  brushes. 

1194.  How  can  one  tell  if  the  resistance  of  the  external  circuit 
is  too  low? 

The  machine  will  build  up  its  voltage  if  the  main  current  is  open, 
as  indicated  in  the  figure ;  but  will  not  build  up  if  the  main  line  is 
closed.  The  reasons  are  given  in  Nos.  1183  and  1184. 

1195.  What  electrical  connections  in  the  shunt  dynamo  are  liable 
to  become  loose  or  broken? 

The  connecting  screws  at  binding  posts  and  elsewhere  are  liable 


DYNAMOS.  287 

to  jar  loose,  and  should,  therefore,  be  tried  about  once  a  week  with  a 
screwdriver  or  wrench.  The  terminals  of  the  fine  wire  of  the  shunt 
field  coils  of  some  machines  are  liable  to  be  easily  broken.  Some- 
times the  connections  between  the  armature  wires  and  commutator 
become  loose  or  broken.  A  frequent  source  of  trouble  is  poor  contact 
between  the  arm  and  segments  of  the  regulating  rheostat;  this  can 
be  tested  while  the  machine  is  running,  but  not  working,  by  touching 
the  ends  of  a  short  wire  to  the  terminals  leading  to  the  rheostat  so 
as  to  short-circuit  it  for  a  moment ;  if  the  trouble  is  in  the  rheostat, 
the  machine  will  come  up  to  voltage  quickly  when  the  rheostat  is 
thus  cut  out.  Sometimes  the  ends  of  the  brushes  get  coated  over  with 
oil  and  dirt  so  as  to  make  poor  contact.  In  some  machines  oil  is  liable 
to  get  into  the  brush  holder  and  connections,  and  so  increase  the 
resistance  of  the  contact  between  the  different  parts.  Trouble  is  less 
likely  to  occur  from  poor  contacts  if  the  machine  is  kept  clean  and 
all  connections  tight. 

1 196.  How  are  improper  connections  liable  to  occur  in  a  dynamo  f 
When  the  machine  is  first  set  up  or  after  repairs,  the  connections 

between  the  armature  and  the  fields  are  liable  to  get  reversed;  the 
field  coils  may  be  connected  so  as  to  oppose  rather  than  assist  each 
other ;  the  machine  may  be  short-circuited.  Short-circuits  are  also 
liable  to  occur  by  accident  or  carelessness  after  the  machine  has  been 
running  all  right. 

1197.  How  can  one  tell  whether  the  different  parts  of  a  dynamo 
are  properly  connected? 

The  simplest  way  is  to  compare  the  connections  with  the  direction 
sheet  and  diagrams  sent  with  the  machine  by  the  maker.  If  these 
are  lacking,  examine  the  machine  and  see  whether  it  is  series,  shunt 
or  compound-wound.  Then  connect  the  field  coils  in  series  or  in 
shunt  with  the  outside  circuit  as  the  case  may  be. 

1 198.  How  can  one  tell  whether  a  machine  is  series,  shunt  or  com- 
pound? 

The  name  plate  on  the  machine  often  tells.  When  it  gives  -both 
the  volts  and  amperes,  the  machine  is  fpr  constant  potential  and  is 
either  shunt  or  compound-wound.  When  the  name  plate  gives  sim- 
ply the  current,  as  6.8  or  9.6  or  10  amps.,  the  machine  is  intended  for 
constant  current,  and  the  field  coil  is  to  be  connected  in  series  with 
the  armature.  Another  method  is  to  examine  the  size  of  wire  on  the 
armature  and  field.  If  the  field  wire  is  larger  than,  or  the  same  size 
as,  the  armature  wire,  the  field  and  armature  are  intended  to  carry 
the  same  current  and  should  be  connected  in  series,  as  indicated  in 


288 


ELECTRICAL   CATECHISM. 


No.  1 140.    If  the  field  wire  is.  much  smaller  than  the  armature  wire, 
the  field  is  intended  to  carry  only  part  of  the  whole  current,  and  it 


FIG.  1198.-COMPOUND  DYNAMO. 

should  be  connected  in  shunt  with  the  main  circuit,  as  indicated  in 
No.  1146. 

1199.  How  can  one  tell  ivhether  the  connections  betzveen  the 
armature  and  field  coils  are  reversed? 

If  the  machine  picks  up  all  right  when  brought  up  to  full  speed, 
the  connections  are  proper.  If  it  does  not  pick  up,  the  field  coils  may 
be  wrongly  connected.  This  may  be  tested  by  a  voltmeter  connected 
across  the  armature  terminals,  as  indicated  in  Fig.  1146,  or  by  a  com- 
pass needle  placed  a  short  distance  from  one  of  the  pole  pieces  of 
the  dynamo,  in  such  a  position  that  it  no  longer  points  to  the  north. 
If  the  machine  is  up  to  speed  when  the  circuit  through  the  field  coils 
is  open,  the  voltmeter  will  indicate  a  few  volts  due  to  the  residual 
magnetism  of  the  iron  field  frame,  as  explained  in  No.  1172.  If  the 
field  coils  are  properly  connected,  this  initial  voltage  will  send  cur- 
rent through  the  field  coils  in  such  a  direction  as  to  strengthen  them 
and  to  cause  a  higher  voltage  in  the  armature.  On  the  other  hand, 
if  the  field  coils  are  improperly  connected,  the  current  due  to  the 
initial  voltage  weakens  the  field  magnetism  and  so  prevents  the  ma- 
chine from  building  up.  This  will  be  indicated  by  the  initial  voltage 
becoming  less  when  the  field  circuit  is  closed ;  also  by  the  needle  of 
the  compass  being  less  strongly  attracted. 


DYNAMOS.  289 

1200.  When  a  dynamo  has  several  field  coils,  should  the  coils  be 
connected  in  series  or  in  parallel? 

The  different  coils  are  almost  always  connected  in  series  with  one 
another  so  that  the  same  current  passes  through  each  coil.  The  ex- 
ceptions to  this  rule  are  that  the  series  coils  of  a  compound 
dynamo  are  connected  in  series  with  the  main  circuit,  and  the  shunt 
coils  carry  only  part,  or  none,  of  the  current  passing  through  the 
series  coils.  Again,  in  some  large  compound-wound  dynamos,  the 
different  sections  of  the  series  field  coils  are  connected  in  parallel,  or 
may  be  connected  either  in  parallel  or  in  series. 

1201.  What  will  happen  if  some  of  the  field  coils  are  reversed? 
If  half  of  the  coils  oppose  the  other  half,  the  machine  will  not  pick 

up  at  all.  If  one  of  four  or  more  coils  is  opposed  to  the  others,  the 
machine  may  pick  up,  but  the  voltage  will  be  low  and  some  of  the 
brushes  will  spark  badly. 

1 202.  How  can  one  tell  which  coil  is  reversed? 

There  should  be  an  equal  number  of  "north"  and  "south"  poles 
in  the  field.  This  may  be  tested  by  a  pocket  compass.  If  there  are 
more  poles  of  one  kind  than  the  other,  it  indicates  that  some  of  the 
coils  are  reversed.  In  nearly  all  machines  the  poles  should  be  alter- 
nately north  and  south.  An  exception  to  this  rule  was  the  Sperry 
arc  machine,  which  had  two  north  poles  together  and  two  south  poles 
together. 

1203.  In  connecting  up  a  dynamo  for  the  first  time,  how  can  one 
be  sure  to  connect  the  different  coils  in  the  right  direction? 

If  there  is  no  direction  sheet  or  diagram,  or  if  the  terminals  of  the 
different  coils  are  not  made  so  as  to  indicate  the  method  of  connec- 
tion, get  a  few  cells  of  battery  and  a  pocket  compass.  See  if  the 
residual  magnetism  in  the  poles  is  alternately  north  and  south.  Send 
a  small  current  from  the  battery  through  one  of  the  coils  and  note 
which  end  of  the  coil  must  be  connected  to  the  zinc  or  negative  ter- 
minal of  the  battery  in  order  to  strengthen  the  residual  magnetism. 
Mark  with  a  string  or  bit  of  paper  the  negative  end  of  each  coil  as 
thus  found.  Connect  the  marked  end  of  one  coil  to  the  unmarked 
end  of  its  neighbor,  continuing  thus  until  only  two  ends  are  left  as  the 
terminals  of  the  entire  series.  These  are  then  to  be  connected  with 
the  armature,  as  indicated  in  No.  1199. 


290  ELECTRICAL   CATECHISM. 

1204.  How  can  one  tell  by  a  compass  needle  whether  a  magnet 
pole  is  getting  stronger  or  weaker? 

Place  the  needle  near  enough  to  the  pole  so  that  it  is  deflected  con- 
siderably from  pointing  north  and  south.  With  the  compass  remain- 
ing in  the  same  place,  the  needle  will  be  deflected  further  from  the 
north  and  south  line  as  the  magnet  pole  becomes  stronger,  approach- 
ing a  limit  when  the  needle  points  east  and  west.  If  the  needle  is  dis- 
turbed, so  as  to  vibrate  from  side  to  side,  it  will  be  found  that  the 
needle  vibrates  faster  as  the  magnet  pole  gets  stronger,  or  as  the 
needle  comes  closer  to  the  pole. 

1205.  How  may  a  dynamo  lose  its  residual  magnetism? 

By  rough  handling  or  by  long  standing  unused,  the  magnetism  be- 
comes weakened.  Sometimes  the  armature  reaction  due  to  a  heavy 
short-circuit  will  demagnetize  or  even  reverse  the  field  magnet. 
Sometimes,  also,  the  induction  current  due  to  lightning  will  de- 
magnetize or  reverse  the  field. 

1206.  How  may  the  field  magnetism  become  reversed? 

By  lightning  or  short-circuit,  as  mentioned  above  in  No.  1205. 
This  is  liable  to  occur  also  when  charging  batteries,  especially  if  the 
dynamo  is  compound  wound  (see  Nos.  1150  and  1244).  Dynamos 
when  not  working  sometimes  become  reversed  by  the  stray  magnetic 
field  of  other  dynamos  near  by. 

1207.  When  the  residual  magnetism  is  lost,  how  can  one  make 
a  dynamo  pick  upf 

The  field  may  be  temporarily  magnetized  by  current  from  another 
dynamo.  Even  a  few  cells  of  battery  will  sometimes  send  enough 
current  through  the  field  coils  to  set  up  sufficient  initial  magnetism 
to  allow  the  machine  to  build  up.  (Of  course  the  battery  circuit 
should  be  broken  before  the  machine  comes  up  to  full  voltage). 
Sometimes  one  end  of  a  bar  magnet  held  against  one  of  the  pole 
pieces  will  magnetize  the  field  enough  for  a  start. 

1208.  Will  a  dynamo  pick  up  if  the  residual  magnetism  is  re- 
versed? 

Yes.  The  initial  voltage  is  reversed,  the  initial  current  through 
the  field  coils  is  reversed  and  the  machine  builds  up  with  its  voltage 
reversed.  The  current  delivered  by  the  machine  is,  of  course,  re- 
versed, and  trouble  will  occur  if  one  attempts  to  connect  the  dynamo 
in  multiple  with  other  machines  not  reversed.  If  the  machine  is  sup- 


DYNAMOS.  291 

plying  arc  lamps,  they  will  be  reversed  also  and  will  throw  their 
light  upward  instead  of  downward.     (See  Nos.  484  and  485.) 

1209.  What  remedy  should  be  applied  when  a  dynamo  becomes 
reversed? 

Reverse  the  residual  magnetism  by  current  from  another  machine 
or  from  a  battery.  If  this  is  not  convenient,  the  connections  between 
the  machine  and  the  line  may  be  transposed  or  crossed,  so  that  what 
was  formerly  the  positive  terminal  of  the  dynamo,  but  is  now  the 
negative  terminal,  will  be  connected  with  the  negative  line  terminal 
and  vice  versa. 

12 10.  Can  not  the  dynamo  be  reversed  by  reversing  the  connec- 
tions between  the  armature  and  field f 

No,  for  then  the  machine  could  not  build  up.     (See  No.  1199.) 

121 1.  Will  a  dynamo  pick  up  if  the  armature  rotates  in  the  op- 
posit e  direction? 

No.  Because  by  reversing  the  direction  of  rotation,  the  voltage 
due  to  the  residual  magnetic  field  (see  No.  1172)  is  reversed  and 
sends  a  current  through  the  field  coils  in  the  wrong  direction,  so 
that  the  dynamo  can  not  build  up. 

1212.  What  changes  must  be  made  if  it  is  necessary  to  run  a 
dynamo  armature  in  the  opposite  direction  from  what  was  intended? 

The  connections  between  the  armature  and  the  field  terminals  must 
be  reversed  so  that  the  initial  current  will  go  through  the  field  coils 
in  the  right  direction  (see  Nos.  1199  and  1211.) 

1213.  What  is  to  be  done  if  the  brushes  suddenly  begin  to  Hash 
or  spark  excessively? 

Look  first  at  the  ammeter  or  current  indicator  to  see  if  the  machine 
is  delivering  too  much  current.  Then  see  if  the  brushes  make  good 
contact,  since  the  springs  or  brush  holders  sometimes  get  loose. 
Then  see  if  the  commutator  has  a  high  bar,  a  low  bar  or  an  open 
circuit. 

1214.  What  can  be  done  if  the  dynamo  is  overloaded? 

If  the  current  is  more  than  50  per  cent  or  100  per  cent  above  the 
rated  capacity  of  the  dynamo,  and  if  it  continues  more  than  a  few 
minutes,  the  main  switch  should  be  opened.  If  the  machine  is  sup- 
plying lights  whose  sudden  extinction  might  cause  a  panic  and  en- 
danger human  life,  one  should  risk  injury  to  the  dynamo  rather  than 
to  life.  When  it  is  unsafe  to  shut  down,  the  dynamo  may  be  relieved 
by  changing  the  regulator  so  as  to  lower  the  voltage  until  the  lamps 
give  only  a  dim  light.  The  current  will  decrease  with  the  voltage. 
Modern  machines  will  carry  50  per  cent  overload  two  hours  safely. 


292  ELECTRICAL  CATECHISM. 

1215.  How  can  one  avoid  an  excess  of  current? 

All  parts  of  the  circuit  should  be  protected  by  safety  fuses  of 
suitable  capacity  so  that  any  branch  taking  too  much  current  will  be 
cut  off  automatically  by  the  melting  of  the  fuses.  Dynamos  are 
commonly  protected  by  circuit  breakers.  (See  Nos.  421  to  440.) 

1216.  If  a  plant  has  no  ammeter  or  current  indicator,  how  can 
the  attendant  tell  when  a  dynamo  is  delivering  too  much  current? 

In  a  well  made  modern  dynamo,  no  part  should  get  too  hot  to  be 
touched  by  the  hand  after  the  dynamo  is  shut  down.  If  the  armature 
begins  to  smoke,  or  if  it  smells  strongly  of  shellac,  it  is  carrying  an 
unsafe  current.  So  long  as  the  armature  does  not  get  uncomfortably 
hot  during  the  regular  run,  the  load  is  within  safe  limits.  With 
some  experience  one  can  get  an  idea  of  the  temperature  of  the  arma- 
ture by  feeling  the  hot  air  coming  from  the  armature. 

1217.  Can  two  arc  dynamos  be  coupled- together  so  as  to  work  in 
series  on  the  same  circuit? 

This  is  frequently  done.  The  two  machies  are  connected  in  series 
so  that  the  same  current  passes  through  each  and  the  voltage  of  one 


DYNAMO    NO.  l  DYNAMO   no.i 

FIG.  1217.— TWO  ARC  DYNAMOS  IN  SERIES. 

is  added  to  that  of  the  other.  Connect  the  positive  terminal  of  No.  I 
to  negative  terminal  of  No.  2 ;  connect  the  positive  line  terminal  to 
positive  terminal  of  No.  2 ;  connect  the  negative  line  terminal  to  nega- 
tive terminal  of  No.  I,  as  indicated  in  the  figure. 

1218.  What  advantages  are  there  in  coupling  two  arc  dynamos 
in  series? 

Coupling  two  machines  in  series  gives  a  higher  voltage,  and  so 
allows  more  arc  lamps  to  be  connected  on  one  circuit.  By  thus  using 
one  long  circuit  instead  of  two  or  more  shorter  ones,  considerable 
wire  is  saved  in  returning  to  the  station,  and  so  less  electrical  energy 
is  lost  on  the  line.  Again,  the  circuits  may  become  so  loaded  that 
they  are  not  even  loads  for  the  dynamos,  and  some  machines  would 
carry  less  than  full  load  while  others  would  have  an  overload.  By 
connecting  several  circuits  and  several  dynamos  in  one  circuit,  it  is 
often  possible  to  arrange  that  each  dynamo  works  at  full  load. 


DYNAMOS. 


293 


1219.  7s  it  true  that  two  arc  dynamos  of  equal  size  connected  in 
series  can  operate  more  than  twice  as  many  lamps  than  one  alone ? 

This  is  sometimes  found  to  be  true.  It  is  probably  due  to  the  fact 
that  in  a  long  circuit  with  many  lamps,  the  fluctuations  due  to  the 
feeding  of  the  lamps  become  more  uniform  as  a  smaller  proportion 
are  feeding  at  the  same  time.  This  allows  the  dynamos  to  work  with 
less  margin  for  fluctuation. 

1 220.  Is  it    not  dangerous  to  connect  several  arc  machines  in 
series? 

It  is  to  some  extent.  Of  course,  the  voltage  is  higher  and  it  is 
necessary  to  have  the  lines,  lamps  and  machines  well  insulated.  The 
liability  to  getting  a  serious  or  fatal  shock  is  increased,  but  no  one 
has  any  business  handling  an  arc  circuit,  unless  he  uses  proper  pre- 
cautions, and  to  most  people  it  would  make  little  difference  whether 
the  shock  was  from  2000  volts  or  10,000  volts. 

1 22 1.  How  are  the  machines  started  when  two  are  coupled  in 
series? 

Make  all  the  connections  and  then  bring  both  machines  up  to  speed 
gradually  and  together.  Or,  if  this  is  not  convenient  or  desirable, 
move  the  brushes  or  regulator  of  each  machine  to  the  position  of  no 
load  and  then  throw  in  both  machines  at  once. 

1222.  How  are  two  or  more  incandescent  dynamos  coupled  for 
combined  output? 

The  two  machines  are  generally  connected  in  parallel  or  in  multiple 


FIG.   1222.— COUPLING    CONSTANT    POTENTIAL   DYNAMOS. 

on  the  same  mains,  as  indicated  in  the  figure.     Both  machines  are 
regulated  to  give  practically  the  same  voltage  and  current. 

1223.  When  two  incandescent  dynamos  are  to  supply  the  same 
mains,  hoiv  may  the  two  be  started  safely? 

This  depends  upon  the  load.  If  it  comes  on  all  at  once,  both  ma- 
chines should  be  started  at  the  same  time.  Connect  both  machines 
to  the  circuit  and  bring  both  up  to  speeed  gradually.  Watch  the 


294  ELECTRICAL  CATECHISM. 

ammeters  and  adjust  the  regulators  so  that  both  machines  may  pick 
up  at  the  same  rate,  and  each  take  its  proper  share  of  the  load.  If  the 
load  comes  on  gradually,  so  that  one  machine  can  carry  it  all  for 
some  time,  another  method  is  used. 

1224.  What  are  bus-bars? 

Bus-bars  are  large  conductors  to  which  a  number  of  machines 
may  be  connected,  'bus  being  an  abbreviation  of  the  Latin  word  "om- 
nibus," meaning  "for  all."  In  large  stations  the  bus-bars  are  com- 
monly built  up  of  a  number  of  flat  strips. 

1225.  How  can  an  incandescent  dynamo  be  safely  coupled  in 
parallel  with  another  one  already  working? 

Bring  the  second  machine  up  to  speed  and  adjust  it  until  it  gives 
the  same  voltage  as  the  one  already  working  and  then  close  the 
switch  so  as  to  connect  the  two  machines  in  parallel.  A  convenient 
way  to  do  this  is  to  have  a  voltmeter  with  a  switch  having  three  or 
more  points  as  shown  in  Fig.  1222.  One  side  of  the  voltmeter  is 
connected  permanently  to  one  of  the  mains,  to  which  also  one  side 
of  each  dynamo  is  connected.  Suppose  the  machine  at  the  left  is 
already  connected  to  the  mains  and  is  at  work,  its  main  switch,  S, 
being,  of  course,  closed.  Move  the  voltmeter  switch  so  as  to  measure 
the  voltage  of  the  line,  or,  what  is  the  same  thing,  the  voltage  of  the 
machine  at  work.  Then  throw  the  switch  over  to  measure  the  voltage 
on  the  fresh  machine.  Adjust  the  rheostat  or  regulator  of  the  fresh 
machine  until  its  voltage  is  just  the  same  or  a  trifle  higher  than  that 
on  the  other  machine.  When  the  voltage  of  the  fresh  machine  is 
just  right,  close  its  switch,  S.  This  should  cause  no  disturbance 
whatever,  and  the  switch  may  be  opened  or  closed  without  any  flash 
or  fluctuation  of  voltage.  After  the  switch  is  closed,  the  regulator 
of  the  fresh  machine  should  be  moved  so  as  to  make  it  pick  up  its 
share  of  the  load,  while  at  the  same  time  the  regulator  of  the  warm 
machine  should  be  moved  in  the  opposite  direction  so  as  throw  off 
some  of  the  load  to  the  fresh  machine. 

1226.  How  can  one  tell  whether  each  machine  is  taking  its  proper 
share  of  the  load? 

Each  machine  should  have  its  own  ammeter  (sometimes  called 
ampere-meter  or  current  indicator)  which  will  show  at  all  times  the 
current  being  supplied.  If  the  machines  are  all  of  the  same  size,  thp 
attendant  should  so  adjust  the- rheostats  and  brushes  that  each  ma- 
chine may  give  the  same  current. 


DYNAMOS.  295 

1227.  Is  it  practicable  or  desirable  to  couple  a  small  dynamo  in 
parallel  with  a  large  one? 

There  is  no  difficulty  at  all  in  doing  this,  except  to  see  that  the 
smaller  machine  does  not  get  more  than  its  full  share  of  the  load. 

1228.  Are   not   incandescent  dynamos  sometimes    coupled    by 
means  of  double-pole  switches  instead  of  single-pole  switches,  as 
indicated  in  Fig.  1222? 

Shunt  dynamos  commonly  have  a  double-pole  switch,  and  com- 
pound dynamos  have  a  three-pole  (or  a  double  and  a  single)  switch, 
so  that  they  are  entirely  isolated  when  not  working.  (See  No.  1251.) 

1229.  Are  not  galvanometers  sometimes  used  instead  of  volt- 
meters for  indicating  when  a  dynamo  is  ready  to  be  thrown  into 
circuit? 

Formerly  a  switch  was  arranged  so  that  a  galvanometer  might 
be  connected  between  the  two  sides  of  the  single-pole  switch  between 
the  dynamo  and  the  main.  The  galvanometer  needle  will  then  be 
thrown  to  one  side  if  the  voltage  of  the  fresh  dynamo  is  too  low,  and 
to  the  other  side  if  it  is  too  high.  When  the  voltage  is  just  right,  the 
galvanometer  needle  will  not  be  deflected.  This  is  now  obsolete. 

1230.  What  will  happen  if  the  switch  is  closed  before  the  fresh 
machine  is  quite  up  to  the  voltage  on  the  line? 

The  fresh  machine  will  take  current  from  the  line,  running  as  a 
motor  and  increasing  the  load  on  the  other  dynamo.  This  may  not 
do  anything  worse  than  cause  the  voltage  on  the  line  to  drop  to  some 
extent,  but  it  is  not  a  good  thing  to  do. 

1231.  When  a  dynamo  runs  as  a  motor,  does  it  not  run  backzvard, 
revolving  in  the  opposite  direction ? 

Not  if  it  is  an  incandescent  dynamo.  Arc  dynamos  are  usually  series 
wound,  that  is,  with  the  field  coil  in  series  with  the  line ;  and  a  series 
motor  runs  in  the  opposite  direction  from  a  series  dynamo.  But  in- 
candescent machines  are  usually  shunt  machines  or  sometimes  com- 
pound wound.  A  shunt  machine  runs  in  the  same  direction  whether 
as  a  dynamo  or  as  a  motor.  A  compound  machine  would  run  in  the 
same  direction  as  a  motor  unless  the  current  were  very  heavy,  when 
it  might  stop  and  then  start  in  the  opposite  direction. 

1232.  //  tzvo  machines  are  working  in  parallel  on  the  same  cir- 
cuit so  that  both  give  the  same  voltage,  how  is  it  possible  to  regulate 
the  division  of  load  between  them? 

This  results  from  the  fact  that  the  voltage  at  the  terminals  of  the 
dynamo  is  somewhat  smaller  than  the  total  voltage  induced  in  the 


296  ELECTRICAL  CATECHISM. 

armature  wires.  There  is  unavoidably  more  or  less  resistance  in  the 
armature  wires,  commutator,  brushes  and  connections,  so  that  the 
current  suffers  some  loss  of  voltage  before  reaching  the  external 
circuit,  the  loss  of  voltage  being  equal  to  the  product  of  the  current 
by  the  resistance.  When  two  machines  are  coupled  in  parallel,  the 
voltage  at  the  terminals  must  be  the  same  for  both.  If  the  total 
voltage  of  either  machine  is  increased,  the  current  delivered  by  it 
increases  until  the  lost  voltage  again  equals  the  difference  between 
the  total  and  that  on  the  line.  When  two  machines  are  working  to- 
gether in  parallel,  if  the  total  voltage  of  one  is  increased  while  the 
other  is  not  changed,  the  voltage  on  the  external  circuit  is  raised 
slightly,  so  that  the  difference  between  the  total  and  external  voltage 
of  the  other  machine  becomes  less.  The  result  is  that  the  first  ma- 
chine takes  more  of  the  load  and  the  other  less.  (See  also  No.  1242.) 

1233.  How  may  the  voltage  of  a  shunt  machine  be  regulated? 
This  is  usually  done  by  adjusting  the  rheostat,  which  is  a  variable 

resistance  connected  with  the  circuit  of  the  shunt  field  magnet  coil, 
as  shown  in  Fig.  1222.  As  resistance  is  cut  out  of  the  rheostat, 
more  current  passes  through  the  field  magnet  coil  and  increases  the 
amount  of  magnetism,  which  correspondingly  increases  the  total 
voltage  induced  in  the  armature.  Another  method  which  can  be  used 
to  a  limited  extent  with  most  incandescent  dynamos  is  to  move  the 
brushes  slightly  forward  or  backward,  so  long  as  they  do  not  spark. 

1234.  Hozv  can  one  machine  be  thrown  out  of  circuit  when  the 
load  drops  off  so  that  the  load  can  be  handled  by  the  remaining  ma- 
chine? 

Move  the  rheostats  and  brushes  of  both  machines  so  as  to  shift  the 
load  from  one  machine  to  the  other  without  disturbing  the  voltage  on 
the  mains,  as  explained  in  No.  1232.  When  all  the  current  is  off 
from  one  machine,  its  switch  may  be  opened  and  the  machine  may 
then  be  stopped. 

1235.  How  can  both  machines  be  shut  down  at  once  if  the  load 
does  not  go  off  so  that  one  machine  can  handle  it? 

If  both  dynamos  are  driven  by  the  same  engine,  the  simplest  way 
is  to  shut  off  steam  slowly  and  let  both  dynamos  and  engine  die  to- 
gether. If  this  is  not  practicable,  move  both  rheostats  around  gradu 
ally  to  the  position  of  lowest  voltage,  watching  both  ammeters,  and 
being  careful  that  the  voltage  drops  equally  on  both,  and  then  open 
both  main  switches  at  the  same  time. 


DYNAMOS.  297 

1236.  What  will  happen  if  one  attempts  to  couple  two  dynamos, 
one  of  which  has  become  reversed? 

This  will  make  a  bad  short-circuit  and  will  melt  the  fuses  on  both 
machines,  besides  burning  the  brushes  and  commutators  badly.  It 
is  also  liable  to  strip  the  armature  wires  out  of  place  and  to  break 
the  belts  or  throw  them  off. 

1237.  How  can  one  tell  whether  a  dynamo  is  reversed  or  not? 
The  voltmeter  will  usually  show  it.    If  the  voltmeter  is  not  one  of 

the  variety  with  a  permanent  magnet,  it  will  not  indicate  polarity. 
If  the  voltmeter  is  connected  as  shown  in  Fig.  1222,  it  will  indicate 
double  voltage  when  connected  to  the  dynamo  which  is  reversed. 

1238.  How  can  one  correct  a  dynamo  that  is  reversed? 

Lift  the  brushes  and  then  close  the  main  switch,  so  that  current 
from  the  mains  will  flow  through  the  magnet  coil  and  magnetize  the 
field  properly.  Then  move  the  rheostat  to  the  position  of  lowest 
voltage  so  as  to  reduce  the  current  through  the  fields ;  open  the  switch 
slowly;  then  place  the  brushes  in  position  and  let  the  machine 
build  up.  This  method  may  also  be  applied  to  a  new  dynamo  that 
will  not  build  up.  If  current  can  not  be  taken  from  mains,  a  few 
cells  of  battery  will  usually  supply  enough  current. 

1239.  Why  should  the  field  switch  be  opened  slowly  after  mag- 
netising a  dynamo? 

This  to  avoid  danger  to  the  insulation  of  the  field  coils  on  account 
of  the  high  voltage  induced  in  them.  When  the  current  ceases  to 
flow  through  the  field  coils,  the  machine  begins  to  lose  its  magnetism, 
and  the  number  of  magnetic  lines  of  force  through  the  field  coils 
diminishes  correspondingly.  This  induces  an  E.M.F.  in  the  coils  in 
such  a  direction  as  to  tend  to  keep  up  the  magnetizing  current.  The 
induced  E.M.F.  is  proportional  to  the  number  of  turns  of  wire  in  the 
coils,  and  also  to  the  rapidity  with  which  the  magnetic  field  changes 
strength.  It  follows  then  that  when  the  current  through  a  shunt  field 
coil  of  many  turns  of  wire  is  cut  off  suddenly,  an  E.M.F.  is  caused 
that  may  be  several  times  higher  than  the  ordinary  working  voltage, 
and  so  may  cause  a  spark  to  jump  through  the  insulation,  and  thus 
make  connection  between  the  coil  and  the  frame  of  the  machine,  or 
short-circuit  part  of  the  coil.  By  putting  in  all  the  resistance  of  the 
rheostat  and  then  opening  the  switch  slowly,  the  current  dies  out 
gradually  and  the  induced  E.M.F.  becomes  comparatively  harmless. 


298  /ELECTRICAL  CATECHISM. 

1240.  Why  does  the  voltage  of  a  shunt  dynamo  fall  off  after  it 
has  been  working  for  some  time? 

This  is  because  the  machine  gradually  becomes  warm  by  use.  The 
resistance  of  the  wire  in  the  field  and  armature  windings  increases 
about  four-tenths  of  I  per  cent  for  each  deg.  C.  rise  of  temperature, 
or  i  per  cent  for  every  5  degs.  of  the  F.  scale.  Therefore,  as  the 
dynamo  warms  up  by  working,  the  resistance  of  the  shunt  field  coils 
increases  and  less  current  passes  through  them.  This  weakens  the 
magnetic  field  and  so  lowers  the  voltage  of  the  dynamo.  In  order 
to  keep  up  the  voltage  of  the  dynamo,  it  is  therefore  necessary  to  cut 
out  some  resistance  from  the  rheostat  in  order  to  balance  the  in- 
creasing resistance  of  the  field  coil. 

1241.  Why  does  a  dynamo  get  warm  by  working? 

It  is  impossible  to  entirely  avoid  friction  at  the  bearings  and  be- 
tween the  brushes  and  commutator.  There  is  also  some  unavoidable 
loss  by  what  is  known  as  "hysteresis"  or  magnetic  friction,  as  the 
magnetization  of  the  armature  is  reversed  twice  for  each  revolution. 
A  further  source  of  loss  comes  from  the  fact  that  the  wire  used  in 
the  dynamo  has  more  or  less  resistance  which  opposes  the  current 
somewhat  as  friction  opposes  motion.  The  work  done  in  sending 
current  through  resistance,  the  work  in  overcoming  the  friction  of 
the  bearings  and  brushes,  and  that  in  overcoming  hysteresis  and  eddy 
current  losses  in  the  armature,  all  this  is  changed  into  heat,  which 
raises  the  temperature  of  the  machine.  (See  No.  415.) 

1242.  Why  does  the  voltage  of  a  shunt  machine  (hop  off  when 
the  load  is  increased? 

This  comes  from  several  causes.  There  is  liable  to  be  more  or 
less  slipping  of  the  belt  as  the  load  increases,  thus  reducing  the 
speed  of  rotation  of  the  armature  and  consequently  lowering  the 
voltage.  There  is  also  more  or  less  resistance  in  the  machine,  part 
of  it  being  in  the  wire,  and  therefore  unavoidable,  and  part  being 
due  to  imperfect  contact  between  the  brushes  and  commutator  and  at 
other  joints  in  the  circuit.  Even  if  the  total  voltage  induced  in  the 
armature  were  not  affected  by  change  of  load,  the  resistance  in  the 
circuit  causes  a  drop  of  voltage  equal  to  the  product  of  current  by 
resistance,  or  the  product  of  amperes  and  ohms.  The  voltage  between 
the  terminals  of  the  shunt  circuit  therefore  becomes  less  than  the 
total  voltage  in  the  armature,  and  consequently  less  current  goes 
through  the  shunt  field  coils.  This  weakens  the  magnetizing  force 
and  in  turn  lessens  the  total  voltage.  Again,  the  current  in  the 
armature  coils  has  a  magnetizing  effect  that  weakens  and  distorts 


DYNAMOS.  299 

the  magnetization  from  the  field  coils.  This  "  armature  reaction  "  is 
proportional  to  the  armature  current  and  helps  lower  the  voltage 
as  the  load  increases.  (See  also  Nos.  1232  and  1258.) 

1243.  How  can  the  voltage  of  a  shunt  dynamo  be  kept  up  as  the 
load  increases? 

It  is  customary  to  have  an  adjustable  resistance  (called  a  rheostat) 
in  series  with  the  shunt  field  coils,  as  indicated  in  Fig.  1146.  Re- 
sistance may  then  be  cut  out  by  moving  the  rheostat  handle  as  de- 
sired. The  voltage  may  also  be  kept  constant  automatically  by  com- 
pounding the  dynamo. 

1244.  What  is  a  compound-ivound  dynamo? 

A  compound-wound  dynamo  has  two  sets  of  field  coils.  (See  Nos. 
1150  and  1151).  As  indicated  in  Fig.  1146,  the  shunt  coil  consists 
of  a  large  number  of  turns  of  comparatively  fine  wire  and  is  con- 
nected to  the  armature  terminals  or  to  the  machine  terminals,  so  as 
.0  form  a  shunt  to  the  external  circuit.  The  shunt  coil  carries  a 
small  fraction  of  the  total  current  and  usually  furnishes  the  larger 
part  of  the  magnetizing  force.  A  compound  dynamo  has  also  a 
scries  coil  of  comparatively  few  turns  of  large  wire,  which  is  con- 
nected between  the  armature  and  the  external  circuit.  The  current 
through  the  series  coil  is  practically  the  same  as  that  in  the  armature 
and  in  the  external  circuit.  The  magnetizing  effect  of  the  series  coil 
is  in  the  same  direction  as  that  of  the  shunt  coil  and  strengthens  the 
magnetization  as  the  load  increases.  The  series  coil  may  be  adjusted 
so  as  to  keep  the  voltage  of  the  machine  constant,  or  it  may  raise  the 
voltage  as  the  load  increases.  In  the  latter  case,  the  machine  is  said 
to  be  "over-compounded." 

1245.  What  is  an  inter-pole  machine? 

An  inter-pole  machine  has  series  wound  auxiliary  poles  between  the 
main  poles,  for  the  purpose  of  balancing  the  armature  cross-magnet- 
izing force,  thus  improving  the  commutation  and  reducing  the  spark- 
ing. Similar  poles  on  motors  allow  wide  variation  of  field  strength 
for  the  purpose  of  changing  the  speed.  (See  Nos.  1193  and  1380.) 

1246.  What  is  meant  by  "short-shunt"  compound  dynamos? 

In  short-shunt  dynamos,  one  terminal  of  the  shunt  field  coil  is  con- 
nected between  the  armature  and  the  series  coil,  as  indicated  in  the 
figure. 

1247.  What  is  meant  by  "long-shunt"  compound  dynamos? 

In  long-shunt  dynamos,  the  terminals  of  the  shunt  field  coil  are 


300 


ELECTRICAL   CATECHISM. 


connected  to  the  outside  terminals  of  the  machine,  as  indicated  in 
the  figure. 


ARMATURE 


.SHORT  SHUNT  DYNAMO  LONG»  SHUNT  DYNAMO 

FIGS.   1246  AND  1247.— COMPOUND    WOUND  DYNAMOS. 

1248  What  are  the  relative  advantages  of  long-shunt  and  short- 
shunt  dynamos? 

Practically  there  is  very  little  difference  between  them,  since  the 
drop  in  voltage  through  the  series  coil  is  very  small.  If  the  machine 
is  to  give  constant  voltage  at  its  terminals  or  at  the  bus-bars  at  the 
switchboard,  it  is  somewhat  simpler  to  calculate  the  number  of  turns 
of  wire  necessary  for  the  series  coil  if  the  shunt  coil  is  connected 
with  the  terminals  and  so  gets  a  constant  current.  Again,  when  sev- 
eral machines  are  operated  in  multiple,  if  they  are  connected  long- 
shunt,  the  shunt  field  coils  may  be  connected  directly  to  the  bus-bars 
at  the  switchboard,  and  so  the  field  of  one  machine  may  be  magnet- 
ized by  those  already  running.  This  enables  one  to  get  a  machine 
ready  for  service  more  quickly  than  when  it  must  pick  up  by  itself. 
It  also  insures  that  the  polarity  of  the  fresh  machine  is  not  reversed. 

1249.  Hozv  are  compound  dynamos  coupled  to  work  in  multiple? 
Compound  machines  act  very  much  like  shunt  machines,  and,  in 

general,  the  explanations  in  Nos.  1222  to  1235  hold  good  for  com- 
pound machines.  With  compound  machines  it  is  necessary  to  have 
an  extra  bus-bar  or  equalizer  making  a  common  connection  for  all 
the  machines  between  the  series  coils  and  the  armatures,  as  indicated 
in  Fig.  1251. 

1250.  Why  is  the  equaliser  necessary  with  compound  machines? 

It  is  to  help  divide  the  load  more  evenly  among  the  different  ma- 
chines. If  the  machines  were  coupled  directly  in  parallel,  the  cur- 
rent from  each  armature  would  go  through  its  own  series  field  coil. 
Thus  it  might  happen  that  when  the  load  was  increased,  if  one  ma- 
chine was  a  little  more  sensitive  than  the  others,  it  would  take  more 
than  its  fair  share  of  the  load ;  this  extra  current  through  its  series 
coil  would  strengthen  its  field  and  cause  it  to  generate  still  more  and 


DYNAMOS. 


301 


more  current  until  its  belt  would  slip  or  its  fuse  would  blow  and 
open  the  circuit.  The  load  would  thus  fail  to  divide  equally  and  the 
operator  would  have  to  adjust  the  rheostats  and  brushes.  Another 
reason  for  using  the  equalizer  is  that,  if  a  dynamo  should  be  thrown 
into  circuit  when  its  voltage  was  lower  than  that  of  the  other  ma- 
chines, or  if  for  any  reason  it  should  afterward  become  lower,  it 
would  take  current  from  the  line ;  this  current  passing  through  the 
^eries  field  coils  in  the  wrong  direction  would  weaken  the  field  and 
so  reduce  the  voltage  of  that  machine  still  further ;  the  result  would 
be  that  the  low  machine  would  run  as  a  motor  and  take  more  and 
more  current  from  the  other  machines. 

1251.     How  does  the  equalizer  help  balance  the  load? 

By  coupling  all  of  the  machines  to  the  equalizer  so  that  all  have 
a  common  connection  between  the  armature  and  the  series  coils,  the 
currents  from  all  the  armatures  unite  and  then  divide  among  the 
different  series  coils.  By  this  means,  if  one  armature  tends  to  deliver 


FIG.    1251.-COMPOUND    MACHINES    WITH    EQUALIZER. 

more  than  its  proportion  of  the  whole  current,  it  strengthens  the 
current  in  the  series  coils  of  all  the  machines,  and  not  its  own  alone. 
On  the  other  hand,  if  one  machine  tends  to  shirk  its  load,  some  of 
the  current  from  the  other  machines  goes  through  its  series  coil  and 
strengthens  its  field  so  that  it  delivers  more  current. 

1252.     How  large  conductors  should  be  used  for  the  equalizer? 

Strictly  the  equalizer  bar  or  bus  should  have  no  resistance,  but 
since  it  is  impossible  to  avoid  all  resistance,  this  should  be  as  small 
as  practicable.  Sometimes  the  equalizer  connections  for  all  the 
dynamos  are  run  to  a  common  point,  so  that  there  is  no  equalizer  bus. 
The  rule  for  determining  the  sizes  of  the  equalizer  connections  is  to 
make  the  resistance  from  the  equalizer  bar  through  the  series  field 
coil  to  the  bus-bar  such  that  each  series  coil  shall  carry  the  same 


302  ELECTRICAL   CATECHISM. 

share  of  the  load  that  its  armature  should  carry.  In  practice,  it  is 
frequently  necessary  to  connect  an  adjustable  resistance  in  parallel 
or  in  series  with  the  series  coil. 

1253.  Does  it  make  any  difference  whether  the  equaliser  is  on  the 
positive  or  negative  side  of  compound-wound  machines ? 

It  makes  no  difference  which  side  of  the  machine  the  equalizer  is 
connected  to  except  that,  of  course,  it  must  be  connected  to  the  same 
side  the  series  field  is  joined  to.  Many  railroad  companies  connect 
the  series  field  and  equalizer  on  the  negative  side  of  the  machine?, 
so  as  to  give  a  low  voltage  between  equalizer  bus  and  ground. 

1254.  Is  it  necessary  to  have  the  equalizer  on  the  negative  side 
of  the  machines  in  order  to  connect  them  with  storage  batteries? 

In  connection  with  storage  batteries,  it  is  only  necessary  to  connect 
the  battery  between  the  equalizer  and  the  opposite  bus,  rather  than 
between  the  outside  bus-bars,  so  that  in  case  the  dynamo  slows  down 
and  the  storage  battery  discharges  back  through  it,  the  current  will 
not  pass  through  the  series  field  and  reverse  the  magnetization  of  the 
machine. 

1255.  Can  compound  machines  be  operated  in  multiple  with 
shunt  machines?    If  so,  how? 

Not  as  a  rule.  It  may  be  possible  to  run  them  for  a  short  time  on 
a  steady  load,  but  it  will  be  in  the  nature  of  a  feat.  The  voltage  of 
the  compound  machine  rises  as  the  load  comes  on ;  the  voltage  of  the 
shunt  machine  falls,  and  this  tends  to  make  the  former  drive  the 
latter  as  a  motor,  unless  rheostats  are  closely  watched.  The  best 
way  is  to  cut  out  the  compound  coils  and  run  all  the  machines  as 
simple  shunt  dynamos.  If  the  compound  machine  is  "under-com- 
pounded" (so  that  its  voltage  does  not  rise  as  the  load  increases), 
the  compound  and  shunt  machines  will  work  together  safely,  but  the 
shunt  machine  will  take  less  than  its  proper  share  of  the  load. 

1256.  Do  the  above  directions  apply  to  alternating  current  gen- 
erators? 

They  apply  to  a  limited  extent  to  alternators,  especially  those  hav- 
ing the  exciter  directly  connected,  or  having  a  commutator  for  com- 
pounding the  field.  They  are  treated  more  fully  in  No.  1427,  etc. 

1257.  Does  the  position  of  the  brushes  have  anything  to  do  with 
the  successful  balancing  of  the  load  among  several  dynamos? 

Very  much.  If  the  field  coils  and  equalizer  connections  are  ad- 
justed for  satisfactory  division  of  the  load,  with  the  brushes  of  all 
the  machines  set  exactly  alike,  it  is  found  that  shifting  the  brushes 


DYNAMOS.  303 

of  any  machine  makes  it  take  more  or  less  than  its  share.  The  closer 
the  brushes  are  to  the  neutral  position,  the  more  current  the  machine 
will  deliver. 

1258.  Why  does  the  position  of  the  brushes  affect  the  sensitive- 
ness of  dynamos? 

With  nearly  all  dynamos  it  is  necessary  to  move  the  brushes  for- 
ward, that  is,  in  the  direction  of  rotation,  when  the  load  increases. 
This  is  because  the  current  flowing  in  the  armature  coils  exerts  a 
magnetizing  influence  which  distorts  and  weakens  the  magnetic  field 
due  to  the  field  coils.  On  account  of  this  distortion,  it  is  necessary  to 
move  the  brushes  forward,  so  as  to  get  them  on  or  near  the  new 
neutral  or  non-sparking  position.  But  the  forward  lead  given  to 
the  brushes  also  increases  the  weakening  magnetic  effect  of  the  arma- 
ture current.  It  follows  that  if  several  similar  machines  are  work- 
ing in  multiple,  an  equal  increase  of  current  in  all  the  armatures 
would  weaken  the  field  of  those  having  greater  forward  lead  more 
than  those  with  less  lead  of  brushes.  Consequently,  the  machine  with 
more  brush  lead  and  weakened  field  would  take  a  smaller  proportion 
of  the  whole  load  than  the  one  with  little  lead.  (See  No.  1232.)  In 
large  stations  it  is  important  to  give  the  same  lead  to  the  brushes  of 
all  the  machines,  so  as  not  to  throw  all  the  load  on  a  few  in  case  of 
short-circuit  or  other  sudden  load. 

1259.  What  is  the  trouble  when  one  or  two  machines  of  a  set  re- 
fuse to  take  their  share  of  the  load? 

It  may  be  too  great  resistance  in  the  series  field  magnet  coil  or  in 
the  equalizer  connections,  so  that  the  magnetic  field  is  not  properly 
strengthened  as  the  load  increases.  (See  Nos.  1244,  1250  and  1252.) 
Too  much  lead  of  brushes  has  the  same  effect  as  just  explained  (see 
No.  1258)  ;  a  dirty  commutator,  loose  or  dirty  connections  in  the 
brush  holder  or  cables,  loose  connection  between  armature  and  com- 
mutator, will  have  the  same  effect  by  increasing  the  resistance  of  the 
armature  circuit.  (See  No.  1232.)  Such  trouble  may  also  be  caused 
by  slipping  of  the  belt,  if  too  loose  or  if  oily  and  slippery.  This 
cause  would  be  indicated  when  the  dynamo  took  its  proper  share 
of  load  until  the  load  became  heavy,  after  which  it  would  take  less 
than  its  share,  or  would  take  the  same  load  regardless  of  the  total 
load. 

1260.  What  is  a  booster? 

When  speaking  of  continuous  currents,  a  booster  is  a  compara- 
tively small  dynamo  connected  into  a  circuit  to. increase  the  voltage 
from  some  other  source.  The  field  is  sometimes  magnetized  by 


304 


ELECTRICAL   CATECHISM. 


coils  in  series  with  the  armature,  and  sometimes  the  machine  is  ex- 
cited from  another  source.  A  booster  is  generally  driven  by  a  motor, 
the  two  armatures  being  directly  coupled,  although  boosters  are 
sometimes  driven  from  the  engine  or  line  shaft.  (See  also  No.  1472.) 

1261.  For  what  is  a  booster  used? 

It  is  used  to  raise  the  voltage  of  one  or  more  circuits  higher  than 
that  on  the  rest  of  the  system.  When  a  number  of  feeders  run  out 
from  the  station,  the  longest  ones  and  those  carrying  the  heaviest 
loads  are  apt  to  have  so  much  drop  on  the  line  that  the  pressure 
at  the  lamps  or  motors  at  the  further  end  becomes  undesirably  low. 
It  would  not  do  to  raise  the  voltage  on  all  the  lines  to  suit  those  that 
were  low,  so  the  economical  remedy  is  to  raise  the  voltage  on  those 
that  have  too  great  drop.  Boosters  are  also  used  in  connection  with 
storage  battery  plants  for  the  purpose  of  raising  the  voltage  of  the 
bus-bars  to  the  amount  necessary  for  charging  the  batteries.  If  the 
battery  can  be  charged  at  a  time  when  the  distributing  circuits  can 
be  cut  off,  or  can  be  handled  by  another  dynamo,  the  batteries  may 
be  charged  directly  from  a  dynamo  without  a  booster  by  simply  rais- 
ing the  voltage  of  that  dynamo. 

1262.  What  are  auxiliary  bus-bars? 

In  order  to  avoid  the  necessity  for  boosters,  some  stations  have  an 
extra  bus-bar,  which  is  kept  at  a  higher  pressure  than  the  main  bus, 
and  to  this  are  connected  the  feeders  that  have  an  extra  large  drop. 


Volts 


Auxiliary  Bus  Bar 


I    Bus   Bars    j_ 


Dynamos 
FIG.   1262.-MAIN  AND   AUXILIARY    BUS-BARS. 

One  or  more  dynamos  maintain  the  pressure  between  the  auxiliary 
bus  and  the  common  negative  bus.  The  figure  shows  the  arrange- 
ment. 


DYNAMOS.  305 

1263.  Does  a  booster  generate  current  in  raising  a  voltage? 

In  a  sense,  it  does.  The  current  flowing  in  the  booster  circuit 
equals  the  total  E.M.F.  (the  sum  of  that  from  the  main  dynamo  plus 
that  due  to  the  booster,  allowing  for  the  C.E.M.F.  of  any  motors  on 
the  line),  divided  by  the  total  resistance  in  the  line.  If  one  so  pre- 
fers, he  may  say,  therefore,  that  part  of  the  current  comes  from  the 
booster,  although  it  must  be  remembered  that  the  whole  current 
passes  not  only  through  the  booster,  but  also  through  the  dynamo. 
Strictly  speaking,  a  dynamo  does  not  generate  current ;  it  sets  up  an 
E.M.F.  which  causes  current  to  pass. 

1264.  What  is  a  motor-generator? 

A  motor-generator  is  a  transforming  device  consisting  of  a  motor 
mechanically  connected  to  one  or  more  generators.  Either  motor  or 
generator  may  be  made  for  direct  current  or  for  alternating  current 
of  any  reasonable  voltage  or  frequency.  Two  alternating  current 
machines  may  be  motor-generators  or  frequency  changers. 

1265.  What  is  a  dynamotor? 

A  dynamotor  combines  both  motor  and  generator  action  in  one 
magnetic  field  with  two  armatures  or  with  two  windings  on  one 
armature. 


FIG.  1265.-DYNA  MOTOR. 

1266.     For  what  purposes  are  dynamotors  used? 

Numbers  of  dynamotors  are  used  In  large  telegraph  offices  for 
supplying  currents  at  various  voltages.  One  armature  winding  is 
designed  for  the  no-volt  lighting  circuit,  while  the  other  winding 
is  suited  to  deliver  current  at  one  of  the  various  voltages  desired. 
Dynamotors  are  sometimes  used  to  obtain  current  for  incan- 


306  ELECTRICAL   CATECHISM. 

descent  lighting  at  no  volts  from  a  power  circuit  at  500  volts. 
Other  machines  are  used  for  obtaining  heavy  current  at  low  voltage, 
such  as  is  required  for  electroplating  or  electrotyping,  current  for 
the  motor  end  being  taken  from  the  regular  circuit  at  no  volts  or 
500  volts.  Others  are  designed  for  charging  automobile  batteries. 

1267.  Is  there  nfft  a  great  loss  in  changing  the  voltage  by  a  dyna- 
motorf 

Not  so  much  as  one  might  at  first  think.  In  a  moderately  small 
machine,  say  of  lo-kw  capacity,  the  efficiency  of  the  motor  part  will 
be  about  90  per  cent,  and  that  of  the  dynamo  end  about  the  same. 
The  combined  efficiency  of  the  combination  will  be  the  product,  or 
81  per  cent.  For  example,  if  10,000  watts  be  supplied  at  the  motor, 
about  8100  watts  may  be  obtained  at  the  dynamo  end. 

1268.  When  a  dynamotor  is  used  to  reduce  the  voltage,  is  it  pos- 
sible to  obtain  a  larger  current  from  the  dynamo  end  than  is  supplied 
at  the  motor  end? 

Yes.  It  is  possible  to  take  out  a  larger  current  than  is  supplied. 
This  is  possible  because  the  larger  current  is  at  lower  voltage  than 
the  current  supplied.  The  power  is  the  product  of  current  by  volt- 
age, and  these  two  might  have  any  values  so  long  as  their  product 
was  less  than  that  of  the  current  and  voltage  supplied  at  the  motor 
end.  For  example,  suppose  the  motor  takes  20  amps,  at  500  volts : 
the  power  taken  is  20  X  500  =  10,000  watts.  The  generator  end 
might  deliver  73.6  amps,  at  no  volts,  making  a  product  of  8100 
watts,  or  8 1  per  cent  of  that  furnished  at  the  motor  end. 

1269.  What  becomes  of  the  difference  "between  the  10,000  watts 
supplied  and  the  8100  watts  delivered? 

It  is  used  up  by  unavoidable  losses  in  the  machine,  such  as  friction 
at  the  bearings  and  brushes,  heating  in  the  armature  wires  due  to 
their  resistance,  the  current  necessary  for  exciting  the  magnetic 
field,  and  heating  of  the  armature  core  due  to  hysteresis  and  eddy 
currents  in  the  iron. 


CHAPTER   XI 


MOTORS. 

(Direct  Current*) 

1300.     What  kind  of  motor  is  the  easiest  to  understand? 

Perhaps  the  simplest  is  the  "oblique  approach"  motors  for  toys  or 
small  sizes  only,  such  as  shown  in  Fig.  13000.  These  must  usually 
be  started  by  hand.  The  moving  and  stationary  parts  are  both  mag- 
netized by  the  same  coil,  and  are  so  designed  that  the  path  of  the 


FIGS.    1300A    AND    1300o.— OBLIQUE   APPROACH    MOTORS. 

magnetic  lines  of  force  becomes  shorter  and  shorter  as  the  armature 
revolves  until  it  reaches  the  shortest  path,  when  the  current  is  cut 
off  for  a  short  time  and  the  attraction  ceases  until  the  momentum  has 
carried  the  armature  past.  Figs.,  b  and  c  illustrate  the  principle  in- 
volved. Suppose  that  the  armature  is  in  the  position  shown  in  Fig. 
b,  when  current  is  sent  through  the  coil  around  the  pivoted  piece, 
N-S.  The  current  sets  up  magnetic  lines  which  extend  through  the 
iron  circuit  somewhat  in  the  direction  shown  by  the  dotted  lines.  The 
tendency  of  these  lines  to  shorten  draws  the  movable  piece  around  to 
the  position  shown  in  Fig.  c.  It  would  stop  here,  because  in  this  po- 
sition the  air  gap  is  shorter  than  in  any  other  position ;  but  a  commu- 


308 


ELECTRICAL   CATECHISM. 


tator  is  arranged  so  that  the  current  is  cut  off  at  this  point  and  the 
moving  piece  keeps  on  moving  by  its  own  momentum,  until  it  reaches 
a  position  one-quarter  turn  ahead  of  the  position  shown  in  Fig.  b, 


FIGS.  1300s  AND  1300c.— OBLIQUE  APPROACH  MOTOR. 

when  current  is  again  sent  through  a  similar  set  of  changes.  Some 
motors  of  this  type  have  the  coils  on  the  stationary  part,  which  has 
usually  more  than  one  pair  of  coils  and  poles,  the  current  going  first 
through  one  pair  and  then  through  the  other,  as  suggested  in  Fig. 
13000. 

1301.  In  the  "oblique  approach"  motors,  which  part  is  the  arma- 
ture and  which  the  field? 

Ordinarily  the  moving  part  would  be  called  the  armature,  and  the 
stationary  part  the  field.  This  is  easy  to  see  in  Fig.  d,  but  there  is 
room  for  discussion  regarding  the  motor  shown  in  Figs,  b  and  c. 

1302.  What  class  of  motors  is  easiest  to  understand  next? 
Those  in  which  the  moving  part  has  two  or  more  projecting  ends 

like  those  on  the  armatures  of  the  "oblique  approach"  motors,  but 
whose  field  magnet  and  armature  have  separate  coils.  Examples  of 


FIG.  1302.— TOY  MOTOR. 


these  are  shown  in  the  accompanying  figures.  The  simpler  one  is  the 
"Edison"  toy  motor,  whose  field  is  a  sort  of  horseshoe  magnet,  and 
whose  armature  is  the  shape  of  the  capital  letter  I  or  H.  The  two 
ends  of  the  coil  upon  the  armature  are  connected  respectively  to  the 


MOTORS.  309 

haives  of  a  split  sleeve,  which  is  carried  upon  the  shaft  and  upon 
which  two  stationary  brushes  rub ;  it  is  evident  that  if  these  brushes 
are  set  properly,  one  brush  will  press  against  one  part  of  the  sleeve 
for  one-half  a  revolution  of  the  armature,  and  upon  the  other  part  of 
the  sleeve  during  the  remainder  of  the  revolution.  If  current  is  sent 
through  the  motor  when  the  armature  is  in  the  position  shown  in  the 
figure,  the  magnetic  lines  of  force  will  pass  through  the  circuit  and  in 
shortening  will  draw  the  armature  to  the  horizontal  position ;  now,  if 
the  brushes  have  been  arranged  properly,  the  commutator  will  have 
turned  around  far  enough  so  that  each  brush  presses  against  a  dif- 
ferent bar  and  the  current  goes  through  the  armature  in  the  opposite 
direction ;  the  armature  and  field  now  repel  instead  of  attract,  and 
when  the  momentum  has  carried  the  armature  past  the  dead  center  it 
will  be  attracted  in  the  other  direction,  and  so  the  rotation  will  con- 
tinue. 

1303.     How  does  the  "Porter"  motor  operate? 

This  motor,  which  is  shown  in  the  figure,  has  three  poles  in  its 
armature.  The  commutator  likewise  is  in  three  sections.  The 
brushes  are  placed  diametrically  opposite,  so  that  they  press  against 


FIG.  1303.— PORTER  MOTOR. 

only  two  of  the  segments  most  of  the  time.  Current  is  sent  through 
the  two  poles  which  are  in  the  position  to  pull  the  hardest,  and  when 
one  pole  gets  to  the  dead  position,  where  it  tends  tc  stay,  current  is 
cut  off  from  it  and  sent  into  another  pole.  This  motor  has  no  dead 
center,  and  will  start  in  any  position. 

1304.     How  are  the  armatures  of  larger  motors  tnadef 

The  iron  core  is  in  the  shape  of  a  cylinder  having  wire  wound  over 

its  entire  surface,  or  in  grooves  upon  the  surface ;  the  wire  is  wound 

in  sections,  which  are  connected  in  series  with  each  other,  and  are 

connected  with  the  successive  bars  of  a  commutator.     A  "smooth 


310 


ELECTRICAL  CATECHISM. 


FIG.    1304A.-CONSTRUCTION   OF   A   TOOTHED   ARMATURE. 


FIG.   1304B.-COMMUTATOR   AND   COMPLETE   ARMATURE. 


MOTORS.  .311 

core"  armature  has  the  wire  upon  the  surface,  while  a  "toothed," 
"slotted"  or  "tunnel  wound"  armature  has  the  wires  in  slots  or  in 
holes  just  below  the  surface.  The  figures  show  the  external  appear- 


FIG.  1304c.— SMOOTH  BODY  DRUM  ARMATURE. 

ance  of  smooth  and  toothed  core  armatures,  and  the  construction  of 
the  latter.  (See  No.  1117.) 

1305.  What  causes  a  surface  wound  armature  to  rotate  when  cur- 
rent passes? 

The  armature  is  in  a  strong  magnetic  field  which  becomes  distorted 
more  or  less  by  the  magnetic  effect  of  the  current  in  each  of  the  wires 
on  the  surface.  The  action  of  each  wire  may  be  considered  as  some- 
thing like  that  shown  in  Fig.  a,  where  there  is  a  strong  pull  tending 
to  move  the  wire  in  between  the  poles  of  the  magnet.  In  the  case 


FTG.  1305B.— MAGNETIC 
FORCES  IN  ARMATURE 


FIG.  I30&A.— REACTION  BETWEEN 
CURRENT   AND   FIELD. 

of  the  actual  armature,  the  lines  of  force  would  naturally  go  into  the 
armature  by  the  shortest  path  through  the  air;  that  is,  would  go 
radially.  But  the  magnetic  force  about  the  current  deflects  the  lines 
and  makes  them  traverse  a  longer  path,  somewhat  as  indicated  in  Fig. 
b,  in  which  circles  represent  wires,  current  going  toward  the  paper 
in  those  with  crosses  and  toward  the  observer  in  those  with  dots. 
The  effort  of  the  lines  to  shorten  causes  the  wires  to  move  in  the  di- 
rection indicated  by  the  arrows  and  thus  rotate  the  iron  armature 
core  to  which  they  are  fastened.  As  the  wires  move  to  such  position 
that  they  would  pull  in  the  opposite  direction,  the  commutator  sec- 


312  ELECTRICAL  CATECHISM. 

tions  to  which  they  are  connected  pass  under  the  brushes,  and  the 
current  is  reversed  and  they  still  pull  in  the  same  general  direction. 

1306.  How  much  is  the  pull  on  a  single  wire  carrying  current 
through  a  magnetic  field? 

The  pull  (measured  in  dynes)  is  found  to  be  equal  to  the  product 
of  the  strength  of  the  field  (measured  in  lines  of  force  per  square 
centimeter)  by  the  strength  of  the  current  (measured  in  the  C.  G.  S. 
unit,  which  equals  10  amps.)  and  by  the  length  of  the  wire  (in  centi- 
meters) which  is  in  the  magnetic  field.  Using  grammes,  centimeters 
and  amperes,  the  pull  in  grammes  equals  the  product  of  the  amperes 
by  the  number  of  lines  of  force  per  square  centimeter  by  the  number 
of  centimeters  length  of  wire  in  the  field  and  divided  by  9,810.  Using 
English  measures,  the  pull  in  pounds  equals  the  number  of  lines  of 
force  per  square  inch  multiplied  by  the  number  of  inches  of  wire  in 
the  magnetic  field  multiplied  by  the  number  of  amperes  through  the 
wire  and  divided  by  11,303,000. 

1307.  How  much  is  the  pull  on  the  zvhole  surface  of  an  armature 
of  a  motor? 

If  the  magnetic  field  is  assumed  to  be  of  uniform  density,  we  may 
calculate  the  pull  on  each  wire  as  above  and  multiply  by  the  number 
of  wires  on  the  armature,  remembering  that  the  current  in  each  wire 
is  only  half  the  whole  current  through  the  armature  of  a  two-pole 
machine,  and  considering  as  the  length  of  each  wire  only  that  part 
that  is  on  the  cylindrical  surface,  neglecting  that  on  the  ends.  An 
easier  method  is  to  consider  the  number  of  lines  of  force  through  the 
armature.  The  number  of  dynes  pull  on  the  surface,  measuring  in  C. 
G.S. units,  equals  the  product  of  the  number  of  lines  of  force  through 
the  armature  by  the  number  of  wires  and  by  the  current  divided  by 
the  circumference  of  the  armature.  The  pull  measured  in  grammes 
equals  the  product  of  lines  of  force  by  amperes  by  number  of  wires 
divided  by  the  circumference  multiplied  by  9810.  The  pull  in 
pounds  equals  the  number  of  wires  by  the  current  by  the  lines  of  force 
divided  by  35,525,000  times  the  diameter  in  inches.  (See  No.  1353.) 

1308.  Give  an  example  of  calculating  the  pull  on  an  armature. 
Take  the  case  of  an  Edison  bipolar  motor  taking  80  amps-  at  117 

volts  running  at  1400  r.  p.  m.,  having  200  armature  wires  and  2,500,- 
ooo  lines  of  force  through  the  armature,  which  is  6£  ins.  in  diameter. 
The  pull  on  the  surface  is  then 

80  X  2,500,000 
P  =  -  =  200  pounds. 

6-5 -X  35>525>ooo 


MOTORS.  313 

1309.  What  is  meant  by  torque? 

By  torque  is  meant  the  pull  at  unit  radius.  In  metric  system  it 
means  the  number  of  dynes  pull  at  a  distance  of  I  cm.  from  the 
center  or  the  number  of  grammes  pull.  In  English  measure  it  means 
the  number  of  pounds  pull  at  a  radius  of  I  in.  or  of  I  ft.  Torque  is 
measured  in  dyne-centimeters,  gram-centimeters,  pound-inches  or 
pound-feet.  In  the  case  of  street  railway  motors  it  is  common  to 
speak  of  the  torque  exerted  on  the  circumference  of  a  wheel  33  ins. 
in  diameter. 

1310.  How  is  the  torque  calculated? 

The  torque  in  pound-feet  equals  the  product  of  armature  wires  by 
amperes  by  lines  of  force  divided  by  852,500,000.  For  example,  the 
torque  in  pound-feet  of  the  above  machine  is 

200  X  80  X  2,500,000 

T  =  —  —  =  48  pound-feet. 

852,500,000 

1311.  Can  the  torque  be  calculated  from  the  power  taken  by  the 
motor? 

It  can,  roughly.  The  above  machine  takes  80  amps,  at  117  volts, 
or  9360  watts,  which,  divided  by  746,  is  12.55  nP>  this  multiplied 
by  33,000  gives  414,150  foot-pounds  as  the  energy  taken  by  the 
motor  and  given  out  by  it  if  there  were  no  losses  in  the  motor.  Since 
it  makes  1400  r.  p.  m.,  the  velocity  at  a  radius  of  i  ft.  is  6.28  times 
1400,  or  8792  ft.  per  minute.  Dividing  the  414,130  foot-pounds  by 
this  velocity  gives  47.1  as  the  pull  at  the  radius  of  T  ft.  This  agrees 
closely  with  the  48  pound-feet  of  the  previous  calculation. 

1312.  How  can  the  torque  of  a  motor  be  actually  measured? 
The  easiest  way  is  to  stop  the  motor,  remove  the  belt,  attach  a 

spring  balance  to  the  pulley  in  such  a  way  as  to  measure  the  pull  at 
its  periphery.  Then,  using  a  rheostat  to  control  the  current,  send  as 
much  current  through  the  armature  as  is  taken  in  regular  working, 
and  measure  the  pull  on  the  spring.  Another  way  is  to  clamp  a  piece 
of  wood  to  the  pulley  and  arrange  a  platform  scales  so  as  to  measure 
the  downward  thrust  when  the  motor  attempts  to  start;  the  scales 
then  weigh  the  pull  at  a  radius  equal  to  the  distance  from  the  center 
of  the  shaft  to  the  point  where  the  lever  touches  the  platform.  There 
are  several  forms  of  transmission-dynamometer  for  measuring  the 
amount  of  power  developed  by  actually  weighing  the  difference  be- 
tween the  pull  on  the  tight  and  on  the  loose  sides  of  the  belt. 

1313.  How  can  the  power  of  a  motor  be  measured? 

A  Prony  brake  is  the  most  convenient  method  for  general  use.    A 


314  -ELECTRICAL  CATECHISM. 

simple  form  suitable  for  short  tests  consists  of  two  blocks  curved  to 
fit  the  pulley  and  having  a  long  arm  and  two  bolts,  as  suggested  in 
the  figure,  and  two  guide  pieces  on  each  side  to  keep  it  from  slipping 
off  sideways.  The  end  of  the  arm  rests  upon  a  sharp  edge,  which  is 


f  Platform  Scales   | 
^         ""          tf 

FIG.  1313.-PRONY  BRAKE. 

supported  upon  the  platform  of  a  scale.  The  scale  measures  the 
torque  at  a  radius  equal  to  the  distance  from  the  center  of  the  shaft 
to  the  knife-edge.  If  in  a  given  case  the  pressure  is  5  Ibs.  at  the  dis- 
tance of  63  ins.,  the  torque  is  315  pound-inches,  or  26.25  pound-feet. 

1314.  Can  the  ivork  done  by  a  motor  be  measured  by  such  a  de- 
vice? 

It  is  only  necessary  to  let  the  motor  run  and  then  measure  the 
pressure  on  the  scale  and  also  measure  the  speed.  The  power  equals 
the  product  of  the  torque  by  the  velocity  at  that  radius.  The  calcula- 
tions are  simplified  if  the  distance  is  63  ins.,  for  then  the  circum- 
ference of  the  circle  passing  through  the  knife-edge  is  33  ft.,  so  that 
I  Ib.  pressure  at  1000  r.  p.  m.  would  mean  33,000  foot-pounds,  or  I 
hp.  In  such  a  measurement  it  is  desirable  to  lubricate  the  blocks  by 
soapy  water  or  by  a  ham-rind,  to  prevent  jerky  action. 

1315.  Give  an  example  of  a  test  on  a  Prony  brake. 

Suppose  the  arm  is  63  ins.  long,  the  pressure  is  8  Ibs.  and  the  speed 
is  1400  r.  p.  m.  Then  the  point  at  the  knife-edge  would  travel  33 
ft.  in  each  revolution,  which  at  1400  per  minute  would  be  33  X  1400, 
or  46,200  ft.  per  minute.  Now,  multiplying  this  by  8  gives  369,600 
foot-pounds  work  done  per  minute.  But  33,000  foot-pounds  make 
I  hp,  hence  dividing  by  33,000  gives  11.2  as  the  horse-power. 

1316.  How  can  the  efficiency  of  a  motor  be  determined? 
Connect  an  ammeter  in  series  with  the  motor  and  a  voltmeter 

across  its  terminals,  so  as  to  measure  the  electrical  energy  supplied. 
Measure  the  mechanical  work  given  out  by  the  Prony  brake.  Reduce 
both  to  horse-power  or  watts  and  divide  the  work  given  out  by  that 
absorbed. 

1317.  Give  an  example  of  measuring  efficiency. 

Suppose  the  motor  measured  in  No.  1314  takes  84  amps,  at  117 


MOTORS.  315 

volts.  The  electrical  energy  taken  is  then  84  X  n/,  or  9828  watts, 
or  13.2  hp.  Dividing  that  delivered  by  that  received  gives  11.2  -r- 
13.2  =  0.85,  or  85  per  cent,  as  the  commercial  efficiency  of  the  mo- 
tor at  that  load. 

1318.  How  can  the  efficiency  at  different  loads  be  determined ? 
By  clamping  the  two  blocks  together  more  or  less  tightly,  the  load 

may  be  varied,  and  so  the  electrical  and  mechanical  power  may  be 
measured  for  different  loads. 

1319.  How  can  one  tell  how  much  power  a  motor  is  giving  out 
when  it  is  in  regular  service  driving  machinery? 

If  the  motor  can  be  cut  loose  from  the  regular  load  and  a  brake 
applied,  the  efficiency  may  be  determined  for  different  currents  as 
above  indicated.  Then  by  measuring  the  current  and  voltage  while 
it  is  working  at  its  regular  load  and  multiplying  by  the  efficiency  of 
the  motor  for  such  load,  we  can  determine  the  power -being  delivered. 

1320.  Of  ivhat  material  is  the  field  magnet  of  a  motor  usually 
made? 

Wrought  iron,  cast  iron  and  cast  steel  are  used.  Wrought  iron 
has  higher  permeability  than  either  of  the  others;  this  allows  the 


FIG.   1320.— MOTOR  WITH   IRONCLAD   FIELD. 

magnetic  field  to  be  made  more  dense,  and  therefore  the  whole  ma- 
chine may  usually  be  made  smaller  for  a  given  power.  Cast  iron 
costs  less  per  pound  and  can  be  cast  into  the  required  shape  more 
cheaply  than  wrought  iron  can  be  forged  or  punched.  Cast  steel, 
containing  but  a  very  small  percentage  of  carbon,  can  now  be  ob- 
tained in  any  desired  form  of  casting,  and  is  rapidly  displacing  cast 
iron  because  its  magnetic  properties  are  almost  as  good  as  those  of 
wrought  iron,  while  it  costs  but  little  more  than  cast  iron.  For  very 


316  ELECTRICAL   CATECHISM. 

small  motors,  hardened  tool  steel  is  sometimes  used,  being  perma- 
nently magnetized  so  that  no  field  magnetizing  coil  is  necessary. 

1321.     How  is  the  field  of  a  motor  usually  magnetized? 

The  hardened  steel  magnets  sometimes  used  with  the  small  motors 
are  magnetized  by  being  placed  between  the  poles  of  a  strong  mag- 
net or  by  passing  a  strong  current  through  a  coil  placed  temporarily 


FIG.  1321. —ARMATURE,   FIELD  COIL  AND  CASTINGS. 

around  the  hardened  steel.  It  is  more  common  to  magnetize  the 
field  by  current  through  a  coil,  which  is  part  of  the  motor,  and 
through  which  part  or  all  of  the  motor  current  passes.  If  all  the 
current  passes  through  the  field  coil,  it  is  called  a  "series  coil."  If 
only  a  small  part  of  the  current  passes  through  the  coil,  it  is  called  a 
"shunt  field  coil."  Sometimes  motors  have  both  series  and  shunt 
field  coils. 

1322.  What  are  the  advantages  and  disadvantages  of  using  per- 
manent magnets? 

The  advantage  is  that  they  do  not  require  a  continuous  expendi- 
ture of  energy  while  the  motor  is  running.  The  disadvantages  are 
that  a  permanent  magnet  can  not  be  made  as  strong  as  an  electro- 
magnet of  the  same  size,  hence  the  motor  has  less  power ;  the  strength  - 
of  such  a  magnet  would  gradually  become  less,  so  that  the  speed  of 
the  motor  for  a  given  voltage  would  increase. 

1323.  Under  what  conditions  are  series  motors  used? 

Small  motors  for  use  with  primary  batteries  are  commonly  wound 
as  series  machines,  likewise  motors  of  any  size  intended  for  use  on 
arc-light  circuits  in  which  the  current  is  of  constant  strength.  Series 
motors  are  also  used  on  constant-potential  circuits,  as  in  railway 
work  and  for  similar  purposes  where  an  attendant  is  always  at  hand 
to  regulate  or  control  the  speed. 


MOTORS.  317 

1324.  What  are  the  advantages  and  disadvantages    of    series 
motors? 

Series  coils  are  somewhat  cheaper  to  wind  than  shunt  coils,  be- 
cause the  wire  is  larger,  costs  less  per  pound  and  less  labor  is  needed 
to  wind  comparatively  few  turns  of  large  wire  than  to  wind  many 
turns  of  small  wire.  This  is  particularly  true  of  small  machines. 
Series  motors  are  more  easily  started,  especially  under  heavy  loads. 
They  maintain  the  speed  more  nearly  constant  than  do  shunt  motors 
when  on  circuits  in  which  the  current  is  maintained  at  constant 
strength.  When  on  constant  potential  circuits,  such  as  used  for  in- 
candescent lighting  or  for  electric  railways,  the  speed  of  a  series 
motor  will  depend  upon  the  load. 

1325.  How  are  series  motors  started  on  battery  circuits? 
Small  series  motors  operated  by  battery  power  are  started  by  sim- 


Field  Armature 

FIG.   1325.— CIRCUIT   OF   SMALL   BATTERY   MOTOR. 

ply  closing  a  switch  to  complete  the  circuit,  as  indicated  in  the  figure. 
The  resistance  of  battery  and  motor  is  sufficient  to  prevent  too  great 
a  rush  of  current  at  the  start. 

1326.     Hoiv  are  series  motors  started  on  arc-light  circuits? 

Motors  of  arc  light  or  "series"  circuits  are  connected  into  the  cir- 
cuit in  the  same  way  as  arc  lamps.  Some  of  them  are  provided  with 
a  simple  single-pole  switch  that  short-circuits  the  motor  when  not 
running,  as  shown  in  the  figure.  To  start  the  motor,  this  switch  is 


FIG.  1326.— SERIES   MOTOR  ON   CONSTANT   CURRENT  CIRCUIT. 

opened,  or  thrown  to  the  "on"  position,  thereby  sending  the  current 
through  the  motor  instead  of  through  the  switch.     No  rheostat  or 


318  ELECTRICAL   CATECHISM. 

resistance  is  needed,  since  the  current  through  the  motor  can  not 
be  greater  at  the  start  than  when  in  regular  running,  this  being  kept 
constant  by  the  dynamo.  It  is  better  to  use  a  double-pole  short-cir- 
cuiting switch,  in  order  to  cut  the  motor  entirely  free  from  the  cir- 
cuit when  not  running ;  in  fact,  this  is  required  by  the  insurance  rules. 
Such  motors  require  the  use  of  high  voltages,  do  not  regulate  satis- 
factorily and  have  gone  almost  entirely  out  of  use. 

1327.     How  are  series  motors  started  on  constant  voltage  circuits? 

It  is  necessary  to  have  some  auxiliary  resistance  in  series,  as  shown 

in  the  figure,  to  prevent  too  great  a  rush  of  current  at  starting.  •  The 


FIG.  1327.— SERIES  MOTOR  ON  CONSTANT  POTENTIAL  CIRCUIT. 

resistance  of  a  I5~hp  railway  motor  is  only  about  2  ohms.  If  this 
were  connected  to  a  5oo-volt  circuit  while  standing  still  and  without 
any  extra  resistance,  the  current  would  be  something  like  250  amps, 
at  the  first  rush,  or  more  than  150  hp.  By  introducing  more  resist- 
ance into  the  circuit  at  first,. the  rush  of  current  is  reduced  and  the 
motor  starts  gradually.  As  the  motor  comes  up  to  speed,  the  extra 
resistance  is  gradually  cut  out.  (See  also  No.  366.) 

1328.  How  are  series  motors  stopped? 

The  operations  in  starting  are  simply  reversed,  care  being  taken 
to  move  the  switches  quickly  in  order  to  avoid  excessive  flashing. 

1329.  On  what  kind  of  circuits  are  shunt  motors  used? 

Shunt  motors  are  used  almost  without  exception  on  constant- 
potential  circuits.  Some  years  ago  the  C.  &  C.  Company  made  a 
motor  for  arc  light  circuits,  having  a  combination  of  series  and  shunt 
coils,  but  this  was  about  the  only  shunt  motor  used  on  such  circuits. 

1330.  What  are  the  advantages  of  shunt  motors? 

They  are  very  closely  self-regulating,  so  that  the  speed  remains 
nearly  constant  for  any  variation  of  load.  They  may,  therefore,  be 
left  to  care  for  themselves  with  only  occasional  inspection  of  bear- 
ings and  brushes.  By  adjusting  the  field  current,  the  speed  may  be 
changed  through  a  range  as  great  as  four  to  one  with  some  motors. 

1331.  What  are  the  disadvantages  of  shunt  motors? 

The  shunt  motor  requires  somewhat  more  care  in  starting  than 
does  a  series  motor.  It  does  not  start  under  a  heavy  load  so  easily 


MOTORS. 


319 


as  do  series  motors.  In  case  the  current  should  be  cut  off  for  any 
reason,  and  the  motor  should  not  be  disconnected  from  the  line  before 
the  current  comes  on  again,  the  motor  is  more  liable  to  be  burnt  out 
than  would  be  the  case  with  a  series  motor. 

1332.     How  is  a  shunt  motor  started? 

A  shunt  motor  is  generally  provided  with  two  switches,  as  indi- 
cated in  the  figure.  The  double-pole  switch,  A,  is  first  closed,  thus 
sending  current  through  the  field  coil,  B.  The  arm  of  a  rheostat,  D, 


— vwwvw — 

o 


FIG.   1332.-CIRCUITS   OF   SHUNT   MOTOR. 

is  then  moved  slowly,  so  as  to  allow  a  moderate  amount  of  current 
to  pass  through  the  armature,  C.  As  the  armature  begins  to  revolve 
and  to  come  up  to  speed,  the  resistance  is  gradually  cut  out  from  the 
rheostat,  until  it  is  all  out  when  the  arm  rests  upon  the  last  button. 

1333.     How  is  a  shunt  motor  stopped ? 

The  double-pole  main  switch  should  be  opened  first,  and  as  soon 
as  the  motor  has  come  to  rest,  the  rheostat  arm  should  be  moved 
back  to  the  "off"  position.  If  the  rheostat  arm  is  moved  back  first,- 


320 


ELECTRICAL   CATECHISM. 


there  is  apt  to  be  vicious  sparking  when  the  -last  point  is  reached  ; 
also  when  the  main  switch  is  opened,  there  is  apt  to  be  a  long  arc 
that  endangers  the  insulation  on  the  field  coils.  Some  of  the  auto- 
matic starting  rheostats  are  arranged  to  take  care  of  themselves  so 
that  one  needs  only  to  pull  the  main  switch. 

I334-  What  is  meant  by  an  "automatic  rheostat?" 
This  expression  may  refer  to  any  of  several  different  devices.  The 
more  common  use  is  with  reference  to  motor  starting  rheostats  which 
have  some  magnetic  device  to  throw  the  starting  resistance  into 
series  with  the  armature  in  case  the  line  voltage  is  removed  for  any 
cause.  Such  are  called  "automatic  motor  starting  rheostats  with 
'no  voltage'  release."  Others  have,  in  addition,  a  device  to  throw  the 
starting  resistance  into  the  armature  circuit  and  thus  stop  or  slow 
down  the  motor  in  case  of  excessive  overload.  The  term  is  also  ap- 
plied to  starting  devices  in  which  the  resistance  is  cut  out  of  the 
armature  circuit  automatically  without  the  direct  control  of  the 
operator. 

T335-     How  are  automatic' motor  starting  rheostats  arranged? 
The  rheostat  arm  carries  a  spring  which  tends  to  throw  it  back  co 
the  "off"  position,  also  the  armature  of  an  electromagnet,  whose  coii 


Release 


FIG.   1335.— MOTOR  RHEOSTAT   WITH   "NO   VOLTAGE"   RELEASE. 

is  in  series  with  the  shunt  field  coil  of  the  motor.     As  soon  as  thr 
current  is  thrown  on  at  the  main  switch,  both  the  motor  field  magne: 


MOTORS. 


321 


and  the  rheostat  release  magnet  are  energized;  and  the  rheostat  arm 
is  magnetically  held  in  position  when  it  has  been  moved  to  the  run- 
ning position.  When  the  main  switch  is  opened,  or  the  pressure  is 
removed  from  the  line  from  any  external  cause,  the  current  ceases 
flowing  around  the  release  magnet,  so  that  it  lets  go  and  the  spring 
throws  the  arm  back  to  the  "off"  position.  This  is  called  a  "no- 
voltage"  release.  In  the  form  shown  in  Fig.  1335,  the  rheostat  arm  is 
held  or  released  by  a  projecting  hook  that  engages  with  the  pivoted 
armature  of  the  electromagnet.  (See  also  Fig.  1341.) 

1336.  Why  does  the  release  magnet  hold  the  rheostat  arm  after 
the  main  switch  has  been  opened  and  until  the  motor  has  come 
nearly  to  rest? 

This  is  because  the  shunt  motor  runs  in  the  same  direction  that 
the  machine  would  run  as  a  dynamo.  When  the  armature  of  the 
motor  is  revolving  in  the  magnetic  field,  it  generates  an  E.M.F.  just 
as  it  would  if  it  were  working  as  a  dynamo.  When  working  as  a 
motor,  this  is  in  the  opposite  direction  to  that  on  the  line,  and  is 
called  a  "counter  electromotive  force,"  or  "C. E.M.F."  The  instant 
the  current  ceases  to  come  from  the  outside  source,  this  C.E.M.F. 
becomes  active  and  maintains  current  through  the  coils  of  the  field 
and  the  release  magnets,  thus  keeping  up  the  magnetism  until  the 
speed  of  the  machine  falls  off  almost  to  a  stop.  (See  No.  1346.) 


Release  magnet 


FIG.   1337.— AUTOMATIC    MOTOR   SWITCH   WITH   "OVERLOAD"    AND 
"NO   VOLTAGE"   RELEASES. 

1337.     How  are  automatic  motor  starting  rheostats  arranged  to 
open  the  circuit  in  case  of  an  excessive  load? 

They  have  an  auxiliary  magnet  whose  coil  is  in  series  with  the  mo- 


322  ELECTRICAL  CATECHISM. 

tor  armature.  In  case  the  current  becomes  greater  than  is  safe,  the 
"overload"  magnet  lifts  its  armature,  which  either  trips  a  spring 
switch  in  the  armature  or  main  circuit,  or  else  short-circuits  the  "no- 
voltage"  release  magnet,  which  then  lets  the  rheostat  arm  fly  back 
and  open  the  armature  circuit  or  leave  it  closed  through  so  high  a 


FIG.  1337A.-AUTOMATIC  MOTOR  SWITCH  WITH  "OVERLOAD"  AND 
"NO  VOLTAGE"  RELEASES. 

resistance  that  the  motor  can  run  only  very  slowly.  The  figures  show 
the  "no-voltage"  magnet  and  the  "overload"  magnet  in  a  well-known 
type  of  starting  box. 

1338.  What  causes  the  long  Hash  if  the  field  circuit  is  opened  after 
the  armature  circuit  is  opened? 

This  is  caused  by  self-induction.  When  the  current  through  the 
magnet  coil  begins  to  stop,  the  magnetism  begins  at  once  to  weaken. 
The  change  in  the  number  of  magnetic  lines  of  force  sets  up  or  "in- 
duces" an  E.M.F.  that  is  in  such  a  direction  as  to  tend  to  maintain 
the  current  in  the  same  direction  and  at  the  same  strength  as  before ; 
the  more  rapid  the  change  in  the  magnetization  and  the  larger  the 
number  of  turns  in  the  coil,  the  higher  will  be  the  induced  E.M.F. 
This  tends  to  maintain  the  current  even  through  the  air  gap  as  the 
switch  is  opened.  In  the  case  of  a  shunt  machine,  the  E.M.F.  due  to 
opening  the  field  circuit  is  likely  to  be  many  times  as  great  as  the 
regular  working  voltage,  and  is  liable  to  cause  a  spark  to  jump 
through  the  insulation  and  either  "ground"  or  "short-circuit"  the 
field  coil. 


MOTORS. 


323 


1339.  Why  is  the  flash  prevented  by  leaving  the  armature  cir- 
cuit closed  until  after  the  motor  has  stopped? 

There  are  two  reasons :  first,  because  the  armature  offers  a  path 
of  low  resistance  for  the  induced  current,  and,  second,  because  the 
armature  tends  to  maintain  the  magnetizing  current  until  rotation 
stops. 

1340.  How  does  the  presence  of  a  closed  circuit  prevent  the  flash? 
When  the  field  coil  is  shunted  by  the  armature  or  by  some  other 

path,  such  as  a  pilot  lamp,  the  E.M.F.  of  self-induction  from  the 
dying  down  of-  the  magnetism  causes  a  current  to  flow  through  the 
coil  and  around  through  the  armature  or  lamp  circuit ;  this  current  is 
in  the  same  direction  as  that  which  came  from  the  line,  and  it  there- 
fore tends  to  keep  up  the  magnetization.  Thus,  if  there  is  a  closed 
circuit  the  induced  current  delays  the  demagnetization  and  so  re- 
duces the  induced  E.M.F. 

1341.  Are  automatic  rheostats  ever  arranged  to  prevent  induc- 
tion when  the  line  switch  is  opened,  after  the  automatic  has  opened? 

In  some  starting  boxes  the  end  of  the  field  circuit  is  permanently 
connected  to  the  first  live  segment  or  button  of  the  rheostat,  an  extra 


FIG.  1341.— STARTING  BOX  PROPERLY  CONNECTED. 


button  being  placed  at  the  retaining  magnet  so  as  to  cut  the  resistance 
out  of  the  field  circuit  at  the  last  step.  This  is  shown  diagrammati- 
cally  and  as  actually  made  in  the  accompanying  sketches. 


324 


ELECTRICAL   CATECHISM. 


1342.  How  are  incandescent  lamps  arranged  to  take  up  the  kick 
of  the  Held  coil? 

A  simple  arrangement  is  shown  in  the  sketch,  in  which  one,  two 
or  five  no-volt  incandescent  lamps  are  connected  in  series  between 
the  armature  and  the  field  terminals  of  the  starting  box.  One  lamp 


FIG.  1342.— FIELD   SAFETY  LAMPS. 

is  used  on  a  no-volt  motor  circuit,  two  on  a  22o-volt  circuit  and  five 
on  a  5oo-volt  circuit.  This  arrangement  has  practically  the  same 
effect  as  that  shown  in  the  preceding  number,  and  is  applicable  to 
any  automatic  starting  box. 

1343.  Is  there  any  risk  in  getting  the  motor  starting  box  con- 
nected improperly? 

If  the  heavy  wires  from  the  motor  are  mixed,  as  shown  in  the 
figure,  the  field  circuit  is  shunted  around  the  armature  when  start- 
ing, instead  of  being  connected  across  the  line.  The  result  is  that  the 


FIG.  1343.-STARTING  BOX  IMPROPERLY  CONNECTED. 

field  is  very  weak  when  it  should  be  strong,  and  the  motor  will  re- 
quire an  excessive  current  for  starting  and  may  blow  the  fuses  or 


MOTORS. 


325 


burn  out  the  motor.     The  condition  is  shown  more  simply  in  the 
second  sketch. 

1344.  How  can  shunt  motors  be  started  and  stopped  safely  from 
a  distant  point? 

A  common  method  is  to  place  the  starting  rheostat  and  the  main 
switch  at  the  point  desired  and  to  run  the  two  main  wires  and  the 
field  wires  from  that  point  to  the  motor.  This  often  requires  con- 
siderable extra  length  of  wire,  which  means  additional  cost  in  wiring 
and  additional  loss  on  the  line. 

1345.  How  can  shunt  motors  be  started  automatically  so  that 
the  operator  need  only  close  or  open  a  single  switch? 

Several  devices  are  used.  At  the  left  of  the  figure  is  one  suitable 
for  small  motors;  when  the  main  switch  is  closed,  current  passes 
through  the  field  coils  and  thence  through  a  starting  solenoid  a, 
which  then  attracts  the  iron  plunger  C,  which  in  turn  raises  the  shoe 
EF  so  as  to  make  contact  with  one  after  another  of  the  rheostat 
contacts  B ;  the  solenoid  thus  closes  the  armature  circuit  through  the 
starting  rheostat  and  then  cuts  out  the  resistance  a  step  at  a  time ; 
by  means  of  the  dash  pot  d  and  valve  e,  the  motion  may  be  made 


I 


FIG.  1345.— AUTOMATIC  MOTOR   STARTERS. 


fast  or  slow.  Four  motors  of  25  hp  and  larger,  a  master  solenoid 
energizes  a  series  of  electromagnetically  operated  switches  one  after 
the  other,  and  these  in  turn  cut  out  the  starting  resistance  one  step 
at  a  time ;  the  main  circuit  is  closed  and  opened  by  a  solenoid  circuit- 


326  ELECTRICAL  CATECHISM. 

maker-and-breaker  having  magnetic  blow-outs  for  disrupting  the 
arcs. 

1346.  How  is  the  magnetisation  of  a  shunt  motor  maintained 
after  the  line  current  is  cut  off? 

The  magnetizing  current  is  caused  by  the  E.M.F.  due  to  the  rota- 
tion of  the  armature  in  the  magnetic  field.  When  the  motor  is  driven 
by  current  from  the  line,  an  E.M.F.  is  generated  in  its  armature  just 
as  in  a  dynamo.  When  the  machine  is  running  as  a  motor,  this 
E.M.F.  is  in  a  direction  tending  to  prevent  current  from  passing 
through  the  armature  from  the  line.  When  the  line  current  is  cut  off 
for  any  reason,  the  motor  E.M.F.  sends  current  through  the  field  coil 
in  the  same  direction  as  the  previous  current  from  the  line,  as  indi- 


FIG.  1346.-DIRECTION  OF  CURRENT  IN  SHUNT  MOTOR. 

cated  by  the  arrows  in  the  figure  at  the  right.  (The  direction  of  the 
currents  from  the  line  is  shown  in  the  figure  at  the  left.)  The  motor 
then  acts  as  a  shunt  dynamo,  exciting  its  own  field  until  the  speed 
drops  off.  (See  No.  1336.) 

1347.  Is  the  motor  E.M.F.  of  any  practical  importance? 

It  is  of  great  importance  in  the  operation  of  the  motor.  Early  in- 
ventors tried  to  get  rid  of  it,  but  it  was  soon  found  that  the  motor 
E.M.F.  was  necessary  for  the  efficient  operation  of  an  electric  motor. 

1348.  Why  did  inventors  try  to  get  rid  of  motor  E.M.F.  ? 
Because  it  limits  the  current  through  the  motor.     They  reasoned 

that  the  power  delivered  by  the  motor  was  proportional  to  the 
current  taken.  But  they  found  that  the  motor  E.M.F.  was  in  such 
a  direction  as  to  oppose  the  current  (for  which  reason  it  is  often 
called  a  "counter  electromotive  force,"  or  "C.EM.F."),  and  so 
they  tried  to  make  motors  that  would  not  develop  much  C.E.M.F. 
Such  motors  were  found  to  take  more  current,  as  was  expected,  but 
they  did  not  develop  the  desired  power. 

1349.  Does  Ohm's  law  apply  to  the  current  taken  by  a  motor? 
It  does  when  properly  applied.    The  current  through  the  armature 

of  a  shunt  motor  equals  the  difference  between  line  E.M.F.  and 
C.E.M.F.  divided  by  the  resistance  of  the  motor  armature  (includ- 
ing resistance  of  brushes  and  commutator).  The  current  through 
the  field  of  such  a  motor  is  simply  the  pressure  divided  by  resistance. 


MOTORS.  327 

1350.  Why  do  motors  with  small  C.E.M.F.  develop  but  little 
power? 

Because  the  same  elements  in  a  motor  that  cause  it  to  deliver 
power  are  also  the  elements  causing  the  C.E.M.F.  In  fact,  it  is  now 
understood  that  the  power  given  out  by  a  motor  equals  the  product 
of  the  current  by  the  C.E.M.F. 

I35I=  What  determines  the  power  given  out  by  a  motor f 
The  power  depends  upon  the  pressure  and  current  supplied  to  the 
motor,  the  current  being  regulated  by  the  amount  of  work  put  upon 
the  motor,  as  will  be  explained.  In  order  not  to  get  into  too  great 
complication  at  first,  it  may  be  presumed  that  the  motor  has  a  steady 
load  and  is  working  properly.  The  work  being  done  by  the  motor 
and  the  electrical  energy  taken  from  the  line  may  then  be  analyzed. 

1352.  What  elements  compose  the  power  delivered  by  the  belt  of 
a  motor? 

The  mechanical  power  equals  the  product  of  the  speed  by  the  pull 
on  the  belt.  For  example,  if  the  belt  is  running  at  55  ft.  per  second, 
and  the  pull  on  the  belt  (or  rather  the  difference  between  the  pulls 
on  the  tight  and  loose  sides  of  the  belt)  is  10  Ibs.,  the  power  is  550 
foot-pounds  per  second,  or  i  hp.  Making  allowance  for  small  losses 
in  friction  and  in  heating  the  iron  core  of  the  armature,  the  product 
of  belt  speed  by  pull  equals  the  product  of  the  pull  on  the  armature 
wires  multiplied  by  the  speed  at  which  they  move.  (In  the  case 
of  toothed  armatures,  the  pull  comes  upon  the  teeth  more  than  upon 
the  wires  themselves,  but  the  theory  may  be  extended  without  great 
difficulty  to  cover  that  case.) 

T353-     What  elements  make  up  the  pull  on  the  armature  wires? 

The  pull  on  each  wire  (measured  in  dynes)  equals  the  product  of 
current  by  strength  of  field  by  length  of  wire  in  the  magnetic  field. 
The  pull  upon  the  whole  armature  is  the  product  of  the  number  of 
wires  by  the  current  through  the  armature  by  the  effective  number 
of  magnetic  lines  of  force  through  the  armature  divided  by  3.14 
times  the  diameter  of  the  armature.  (See  also  Nos.  1306,  1307  and 

1308.) 

1354.     What  elements  make  up  the  velocity  of  the  armature  wires? 

The  velocity  equals  the  number  of  revolutions  by  the  distance 
around  the  armature.  The  latter  is,  of  course,  the  circumference  of 
the  armature  (considering  the  average  position  of  the  wires)  and 
is  3.14  times  the  average  diameter. 


328  ELECTRICAL   CATECHISM. 

J355-  What  elements  compose  the  power  delivered  to  the  shaft 
or  pulley  of  the  motor? 

This  equals  the  product  of  the  pull  upon  the  armature  wires  mul- 
tiplied by  their  speed.  In  multiplying  the  factors  of  the  two  ele- 
ments, as  given  in  Nos.  1352  and  1353,  it  is  seen  that  ''3.14  times  the 
diameter"  multiplies  one  factor  and  divides  the  other,  so  that  the 
two  cancel.  The  final  product  for  the  work  is :  Armature  current 
multiplied  by  number  of  armature  wires  by  effective  number  of  mag- 
netic lines  through  armature  by  number  of  revolutions  per  second. 

1356.     What  elements  compose  the  C.E.M.F.? 

The  counter,  or  motor,  E.M.F.  equals  the  rate  of  cutting  lines  of 
force  and  is  the  product  of  the  number  of  armature  wires  by  the 
number  of  revolutions  per  second  by  the  effective  number  of  mag- 
netic lines  of  force  through  the  armature.  (This  gives  the  E.M.F. 
measured  in  the  absolute  or  "C.  G.  S."  unit,  which  is  only  one  one- 
hundred-millionth  part  of  a  volt.) 

J357-  What  is  the  relation  between  power  and  counter  electro- 
motive force? 

From  Nos.  1354  and  1355  it  is  seen  that  the  power  equals  current 
multiplied  by  the  product  of  wires  by  revolutions  by  lines  of  force, 
which  equals  current  by  C.E.M.F.  Thus  it  is  seen  that  while  the 
C.E.M.F.  limits  the  current,  it  is  one  of  the  factors  of  the  power. 
(In  the  case  of  alternating-current  motors,  the  power  of  the  motor 
equals  the  product  of  current  by  the  part  or  component  of  the 
C.E.M.F.  which  is  in  phase  with  current,  this  product  being  multi- 
plied by  another  factor  depending  upon  whether  it  is  a  single  phase, 
two-phase  or  three-phase  system.) 

1358.     For  what  sort  of  a  motor  does  the  above  reasoning  apply? 

For  simplicity,  the  above  has  been  developed  only  for  a  bipolar  di- 
rect-current motor,  but  the  argument  might  be  modified  to  apply  to 
any  kind  of  a  motor,  and  the  general  conclusion  applies  to  any  elec- 
tric motor. 

J359-     What  units  were  used  in  the  above  argument? 

For  simplicity,  the  units  of  the  metric  or  "C.  G.  S."  system  were 
used.  The  conclusion,  however,  in  No.  1356  is  independent  of  the 
units  used. 

1360.     7V  hat  is  meant  by  "effective  magnetic  lines  of  force"? 

In  a  dynamo  or  motor  the  brushes  are  so  set,  or  the  magnetic  field 
is  so  distributed,  that  the  E.M.F.  in  some  of  the  wires  is  in  the  oppo- 
site direction  to  that  in  the  rest.  Sometimes  this  is  to  diminish  the 


MOTORS.  329 

sparking  at  the  brushes,  or  is  due  to  incorrect  design  or  to  improper 
setting  of  the  brushes.  For  this  reason  the  effective  number  of  wires 
or  of  magnetic  lines  is  less  than  the  total.  We  may  consider  either 
that  not  all  the  wires  are  effective,  or  that  not  all  the  lines  are  ef- 
fective. 

1361.  Does  the  power  delivered  by  a  motor  increase  as  the 
C.E.M.F.  increases? 

The  two  vary  inversely  so  long  as  the  C.E.M.F.  is  more  than  half 
of  the  line  E.M.F.  This  is  because  the  current  is  controlled  by  the 
C.E.M.F.,  being  equal  to  the  difference  between  the  line  voltage  and 
the  C.E.M.F.  divided  by  the  resistance  of  the  armature  circuit.  The 
power  equals  the  product  of  current  by  C.E.M.F.,  and  it  follows  that 
the  power  increases  as  the  C.  E.  M.  F.  decreases,  so  long  as  the  dif- 
ference between  the  two  E.M.F's  increases  faster  than  the  C.E.M.F. 
decreases. 

1362.  Give  an  example  to  illustrate  this  point. 

Assume  the  line  pressure  to  be  100  volts,  the  armature  resistance 
to  be  I  ohm  and  the  C.E.M.F.  to  be  90  volts ;  the  current  is  100  minus 
90  divided  by  i,  or  10  amps.;  the  power  delivered  by  the  motor  is 
10  multiplied  by  90,  or  900  watts;  while  the  power  taken  by  the 
motor  is  10  multiplied  by  100,  or  1000  watts.  Now,  suppose  the 
C.E.M.F.  drops  to  60  volts;  the  current  is  100  minus  60  divided  by 
i ,  or  40  amps. ;  the  power  delivered  is  40  multiplied  by  60,  or  2400 
watts.  Suppose  the  C.E.M.F.  is  50  volts ;  the  current  is  50  amps., 
and  the  power  delivered  is  2500  watts.  Suppose  the  C.E.M.F.  is  20 
volts;  the  current  is  80  amps.,  and  the  power  delivered  is  1600  watts. 
It  is  thus  seen  that  the  power  increases  until  the  C.E.M.F.  equals 
half  the  line  E.M.F.,  after  which  the  power  becomes  less,  although 
the  current  continues  to  increase.  (As  a  general  rule  the  safe  cur- 
rent carrying  capacity  of  the  armature  will  be  passed  before  the 
C.E.M.F.  drops  to  50  per  cent  of  the  line  voltage.) 

1363.  What  is  meant  by  the  efficiency  of  a  motor? 

The  efficiency  of  any  machine  is  the  ratio  of  the  power  delivered 
to  that  received.  The  total  power  given  out  by  a  direct-current 
motor  is  the  product  of  the  armature  current  by  the  C.E.M.F.  The 
net  power  delivered  is  something  less  than  this  by  an  amount  equal 
to  the  heat  loss  in  the  armature  core  plus  the  friction  at  the  brushes, 
bearings  and  against  the  air.  The  total  power  received  by  the  motor 
equals  the  sum  of  the  electrical  energies  given  to  the  armature  and 
to  the  field.  We  may  distinguish  between  the  electrical  and  the  com- 
mercial efficiency  of  a  motor. 


330  ELECTRICAL   CATECHISM. 

1364.  What  is  meant  by  the  electrical  efficiency  of  a  motor ? 
The  electrical  efficiency  of  a  motor  is  the  ratio  of  the  product  of 

armature  current  by  C.E.M.F.  to  the  total  electrical  energy  supplied 
to  the  motor.  Neglecting  the  small  amount  taken  by  the  field  we 
may  say  that  the  electrical  efficiency  is  the  ratio  of  the  energies  in  the 
armature ;  that  is,  the  product  of  armature  current  by  total  voltage 
across  the  armature  divided  by  the  product  of  the  armature  current 
by  the  C.E.M.F.  Since  the  current  is  the  same  in  numerator  and 
denominator,  we  may  cancel  it  and  say  that  the  electrical  efficiency 
is  the  total  voltage  divided  by  the  C.E.M.F.  (The  above  refers  only 
to  direct-current  motors,  for  in  A.  C.  motors  the  C.E.M.F.  may  be 
smaller  or  larger  than  the  line  voltage,  and  the  product  must  be  mul- 
tiplied by  a  factor  depending  upon  the  difference  of  phase  between 
current  and  pressure.  (See  also  No.  1357.) 

1365.  What  is  meant  by  the  commercial  efficiency  of  a  motor? 
The  commercial  efficiency  is  the  ratio  of  the  power  delivered  from 

the  pulley  to  the  total  electrical  energy  received. 

1366.  How  can  the  C.E.M.F.  be  measured? 

It  can  not  be  measured  directly.  It  can  be  calculated  by  taking 
the  difference  between  the  total  pressure  at  the  terminals  of  the 
armature  and  the  product  of  armature  current  by  the  resistance  of 
the  armature. 

1367.  Show  how  the  efficiency  of  the  motor  is  affected  by  changes 
of  C.E.M.F.  in  the  above  case  (No.  1362). 

When  the  C.E.M.F.  is  90,  the  energy  taken  by  the  motor  is  the 
product  of  current  by  volts,  10  times  100,  or  1000  watts;  the  power 
delivered  is  900  watts,  making  the  efficiency  of  the  motor  to  be  900 
divided  by  1000,  or  90  per  cent.  When  the  C.E.M.F.  is  20,  the  power 
taken  is  80  times  100,  or  8000  watts,  while  the  power  delivered  is 
1600  watts,  and  the  efficiency  is  20  per  cent.  The  efficiency  is  thus 
seen  to  be  equal  to  the  C.E.M.F.  divided  by  the  line  E.M.F. 

1368.  How  does  the  C.E.M.F.  affect  the  starting  of  a  motor? 

When  a  motor  is  standing  still,  there  is  no  C.E.M.F.  due  to  arma- 
ture rotation,  and  the  current  taken  at  first  is  governed  principally 
by  the  resistance  of  the  circuit.  If  the  motor  is  series  wound,  there 
is  a  transient  C.E.M.F.  due  to  self-induction  for  a  few  seconds  dur- 
ing the  rapid  building  up  of  the  magnetic  field.  If  the  motor  is 
shunt  wound,  self-induction  delays  the  current  through  the  field  coil, 
but  that  through  the  armature  is  not  impeded  much  from  such  cause. 
As  soon  as  the  armature  begins  to  rotate,  the  motor  E.M.F.  begins  to 


MOTORS.  331 

develop  and  the  resistance  may  be  cut  out  gradually  from  the  starting 
box  as  the  armature  comes  up  to  speed.  Thus  C.E.M.F.  gradually 
replaces  ohmic  drop  in  limiting  the  current  as  the  motor  starts. 

1369.  What  governs  the  current  taken  when  the  motor  is  running, 
at  full  speed? 

At  a  given  load  and  speed,  the  current  is  governed  by  the  C.E.M.F. 
and  the  resistance  of  the  armature  circuit  (neglecting  the  small  cur- 
rent through  the  field),  and  the  pull  on  the  belt  is  just  enough  to 
turn  the  shaft  at  the  right  speed.  If  an  additional  load  is  put  on,  the 
pull  on  the  armature  for  an  instant  is  not  large  enough,  and  it  begins 
to  slow  down;  this  reduces  the  C.E.M.F.,  which  allows  greater  cur- 
rent to  pass  and  increase  the  pull  on  the  armature  and  so  bring  the 
armature  up  toward  its  original  speed.  If  the  voltage  on  the  line 
remains  constant  and  if  the  magnetic  field  of  the  motor  is  not 
changed,  the  armature  will  run  somewhat  slower  as  the  load  in- 
creases. 

1370.  Does  the  resistance  of  the  armature  have  much  to  do  with 
the  amount  the  speed  slows  down? 

The  less  the  resistance  of  the  armature,  the  more  nearly  the 
C.E.M.F.  equals  the  line  voltage,  and  the  less  reduction  of  C.E.M.F. 
necessary  to  allow  the  greater  current  to  pass.  Hence  the  speed  of 
the  motor  will  be  more  constant  for  varying  loads  as  the  resistance 
of  the  armature  is  less. 

1371.  How  does  armature  resistance  affect  the  efficiency  of  a 
motor? 

The  efficiency  is  higher  as  the  armature  resistance  is  lower.  The 
voltage  of  the  line  is  balanced  by  the  sum  of  the  C.E.M.F.  and  the 
ohmic  drop  through  the  armature.  The  ohmic  drop  equals  the 
product  of  current  by  resistance.  Hence,  for  a  given  current,  the 
less  the  resistance  the  less  will  be  the  ohmic  drop  and  the  greater  will 
be  the  voltage  left  to  balance  the  C.E.M.F.  As  was  seen  in  No. 
1360,  the  efficiency  is  greater  as  the  C.E.M.F.  is  greater;  hence,  the 
less  the  resistance  of  the  armature,  the  greater  will  be  the  efficiency 
of  the  motor. 

1372.  Can  the  resistance  of  the  armature  be  changed f 

Only  by  rewinding  it.  It  should  be  remembered,  however,  that 
part  of  the  resistance  of  the  armature  circuit  is  at  the  contact  between 
the  brushes  and  commutator.  Therefore,  a  motor  with  dirty  or  un- 
even commutator,  loose  brushes  and  poor  connections  will  not  run 
at  so  uniform  speed  as  the  same  on^  when  put  into  first-class  run- 


332  ELECTRICAL  CATECHISM. 

ning  order.  Any  excessive  heat  around  the  brushes  or  holders  should 
therefore  be  investigated. 

J373-     What  regulates  the  speed  of  a  motor? 

The  speed  of  a  motor  is  governed  by  the  resistance  of  its  armature 
circuit  and  by  its  C.E.M.F.  Enough  current  must  pass  through  the 
armature  (with  a  given  magnetic  field)  to  make  the  torque  or  pull 
equal  to  the  load.  This  current  multiplied  by  the  resistance  of  the 
armature  circuit  gives  a  certain  ohmic  drop.  The  C.E.M.F.  must 
equal  the  difference  between  this  and  the  line  voltage.  With  a 
given  magnetic  field  and  armature,  the  only  variable  element  in  the 
C.E.M.F.  (see  No.  1356)  is  the  speed.  This,  therefore,  must  be 
such  as  to  bring  the  C.E.M.F.  to  such  a  value  that  the  difference  be- 
tween it  and  the  line  voltage  will  just  send  sufficient  current  through 
the  armature  to  give  the  necessary  torque. 

1374.  How  can  the  speed  of  a  motor  be  increased  with  a  given 
load? 

The  speed  of  a  motor  on  a  constant-potential  circuit  can  be  in- 
creased either  by  reducing  the  resistance  in  the  armature  circuit  or 
by  weakening  the  magnetic  field,  that  is,  by  reducing  the  effective 
number  of  magnetic  lines  of  force  through  the  armature, 

1375.  To  what  extent  can  the  speed  be  regulated  by  changing  the 
resistance  of  the  armature  circuit? 

The  speed  may  be  reduced  to  any  amount  desired  by  putting 
enough  resistance  in  series  with  the  armature.  This  is  not  economi- 
cal, however,  for  all  the  pressure  lost  in  the  resistance  (when  multi- 
plied by  the  current)  represents  lost  energy  wasted  in  heating  the 
resistance.  The  speed  increases  as  the  resistance  is  cut  out  from  the 
circuit  and  the  upper  limit  is  reached  when  all  the  adjustable  re- 
sistance is  out. 

1376.  How  can  the  power  delivered  by  a  motor  be  increased? 
The  power  equals  the  product  of  speed  by  torque  or  pull.    Either 

or  both  of  these  may  be  varied  by  changing  either  the  voltage  at  the 
terminals  of  the  armature  or  by  changing  the  effective  number  of 
magnetic  lines  of  force  through  the  armature. 

1377.  How  can  the  effective  number  of  magnetic  lines  of  force 
through  the  armature  of  a  motor  be  changed? 

Either  by  shifting  the  brushes  or  by  changing  the  number  of  am- 
pere turns  in  the  field-magnet  coil.  As  a  general  rule,  it  is  not  ad- 
visable to  shift  the  brushes  far  from  the  neutral  plane,  lest  excessive 
sparking  damage  brushes  and  commutator« 


MOTORS. 


333 


1378.  How  does  a  change  in  the  effective  number  of  magnetic 
lines  of  force  through  the  armature  affect  the  torque  in  the  case  of 
a  motor  on  a  constant-current  circuit? 

The  torque  is  proportional  to  the  product  of  current  by  armature 
wires  by  lines  of  force.  (See  Nos.  1306  to  1309.)  In  a  constant- 
current  circuit,  such  as  an  arc  light  line,  the  effective  number  of  ar- 
mature wires  or  the  effective  number  of  lines  of  force  may  be  varied 
to  some  extent  by  shifting  the  brushes.  The  total  number  of  lines 
of  force  may  be  varied  by  changing  the  number  of  ampere  turns 
through  the  field  coil.  The  latter  may  be  varied  either  by  connecting 
a  shunt  of  variable  resistance  around  the  field  coil  or  by  having  a 
number  of  terminals  so  that  the  part  of  the  coil  may  be  cut  out. 


FIG.  1379.— MOTOR  WITH   VARIABLE   FIELD. 

1379-  How  does  a  change  in  the  field  affect  the  torque  of  a  motor 
on  a  constant-potential  circuit? 

The  effect  in  such  a  case  is  more  complicated  than  in  the  preceding 
case,  for  a  change  in  the  field  affects  the  current  and  both  the  speed 
and  the  torque  are  apt  to  change  in  consequence.  With  a  given  cur- 


334  ELECTRICAL   CATECHISM. 

rent,  the  torque  would  be  increased  by  strengthening  the  field;  but 
the  stronger  field  increases  the  C.E.M.F.  for  the  same  speed,  and 
this  reduces  the  current.  Under  ordinary  conditions  it  is  found  that 
strengthening  the  field  of  a  motor  on  a  constant-potential  circuit 
causes  a  more  than  proportional  decrease  of  current.  The  result  is 
that  within  the  limits  met  in  ordinary  practice,  the  torque  and  the 
power  of  a  motor  on  constant-potential  circuit  are  actually  increased 
by  weakening  the  field. 

1380.  How  does  a  change  in  the  field  affect  the  speed  of  a  motor 
on  a  constant-potential  circuit? 

Weakening  the  field  causes  the  armature  to  run  faster.  Suppose 
a  motor  has  a  load  that  takes  the  same  belt  pull  regardless  of  the 
speed.  Since  the  field  is  weakened,  the  current  must  increase  to 
maintain  the  torque  constant.  This  increased  current  causes  a 
greater  ohmic  drop  and  so  reduces  to  some  extent  the  C.E.M.F. 
necessary  to  equal  the  balance  of  the  line  pressure.  Since  the  field 
is  weaker,  the  speed  must  be  increased  proportionally  to  bring  the 
C.E.M.F.  to  its  former  value  and  almost  that  much  to  bring  the 
C.E.M.F.  to  the  new  and  slightly  smaller  value.  It  is  difficult  to 
show  the  exact  relationship  without  using  algebra,  but  the  conclu- 
sion is  that  for  the  same  belt  pull  the  speed  is  increased  by  weaken- 
ing the  field.  A  range  of  four  to  one  is  sometimes  practicable. 

1381.  What  is  the  common  method  of  regulating  motors? 
Motors  on  constant-potential  circuits  are  generally  plain  shunt 

motors.  When  it  is  desired  to  adjust  the  speed  accurately,  a  rheostat 
is  cut  into  the  field  circuit.  In  some  cases  the  motor  is  compound 
wound ;  that  is,  it  has  two  field  coils,  one  of  fine  wire  shunted  around 
the  armature  and  one  of  coarse  wire  in  series  with  the  armature. 
The  series  coil  may  be  cumulatively  connected  so  as  to  assist  the 
shunt  coil  and  strengthen  the  field  as  the  load  increases,  or  it  may  be 
connected  differentially  so  as  to  oppose  the  shunt  coil  and  weaken 
the  field  as  the  load  increases. 

1382.  Under  what  circumstances  are  the  series  coils  connected 
differentially? 

The  differential  series  coils  may  be  so  adjusted  as  to  maintain  the 
speed  constant  at  all  loads  by  weakening  the  field  just  enough  so 
that  the  C.E.M.F.  at  the  desired  speed  falls  just  enough  to  equal 
the  increased  ohmic  drop  due  to  the  greater  current  at  the  greater 
load. 


MOTORS.  335 

1383.  Under  what  conditions  are  the  series  field  coils  connected 
cumulatively? 

The  series  coils  are  connected  so  as  to  strengthen  the  field  due  to 
the  shunt  coils  when  the  motor  must  start  under  heavy  load  or  when 
the  load  is  subject  to  great  fluctuations  and  when  exact  regulation  of 
speed  is  of  less  importance  than  disturbances  of  the  line  pressure. 
In  some  cases  the  armature  reaction  weakens  the  field  so  much  that 
the  series  coils  must  be  connected  cumulatively  to  keep  the  field 
strong  enough  and  so  maintain  constant  speed. 

1384.  Why  do  some  plain  shunt  motors  run  at  almost  constant 
speed  at  any  and  all  loads? 

By  making  the  motor  with  very  strong  field  the  armature  wires 
may  be  made  fewer  and  consequently  of  lower  resistance.  For  rea- 
sons given  in  Nos.  1369  to  1372,  the  motor  regulates  more  closely 
as  the  armature  resistance  is  less.  Some  motors  also  are  so  designed 
that  the  magnetizing  force  of  the  current  through  the  armature  coils 
weakens  the  field  just  enough  to  maintain  the  speed  constant  at  all 
loads. 

1385.  Why  are  series  motors  used  on  street  railways? 
Because  of  their  great  starting  power,  the  ease  of  controlling  the 

speed,  the  smaller  cost  of  winding  the  field  magnet  coils. 

1386.  Why  do  series  motors  have  greater  starting  power  than 
shunt  motors? 

Since  the  whole  of  the  armature  current  passes  through  the  field 
coils,  the  magnetic  field  is  strongest  when  the  armature  current  is 
greatest.  This  occurs  at  the  time  of  starting,  for  when  the  motor  is 
standing  still  the  C.E.M.F.  is  small.  The  torque  is  very  great,  since 
both  the  field  and  the  current  through  the  armature  are  strong.  Re- 
sistance is  placed  in  series  with  the  motor  to  prevent  too  great  a  rush 
of  current,  which  would  start  the  car  with  an  uncomfortable  jerk.  As 
the  car  begins  to  move,  the  resistance  is  rapidly  cut  cut  and  C.E.M.F. 
replaces  ohmic  drop  in  regulating  the  current  taken  by  the  motor. 

1387.  What  governs  the  final  speed  of  a  series  motor  when  pull- 
ing a  car? 

As  the  speed  of  the  car  increases,  the  C.E.M.F.  of  the  motor  in- 
creases for  a  given  current.  This  reduces  the  current  and  conse- 
quently the  field  strength,  both  of  which  reduce  the  torque.  The 
torque  of  the  motor  is  used  partly  in  accelerating  the  car  and  partly 
in  overcoming  the  more  or  less  constant  resistance  of  the  track.  As 
the  speed  increases,  the  current  through  the  motor  decreases  and  its 


336 


ELECTRICAL   CATECHISM. 


torque  decreases  until  a  balance  is  reached  when  the  motor  torque  is 
just  sufficient  to  maintain  the  speed  of  the  car.  The  relationship  of 
current,  strength  of  field,  C.E.M.F.  and  torque  is  complicated  in 
such  a  case.  In  some  cases  the  speed  is  further  increased  by  shunt- 
ing part  of  the  current  around  the  field  coils  or  by  cutting  out  part  of 
the  coils ;  in  modern  practice  it  is  common  to  connect  two  motors  in 
series  at  the  start,  so  that  each  is  subject  to  only  half  the  line  voltage ; 


A — Efficiency  without  Gears. 

B — Approximate   Efficiency  with   Gears. 

C — Safe  Time  for  Load  in  Service  20° 

C.  rise  from  75°   C. 
D— Speed  M.   P.  H. 
E— Electrical  HP. 
F— Brake  HP.  with  Gears. 
G — Tractive  Effort. 
H — Time  to  Rise  75°  C.  from  25°  C. 


FIG.  1385.— RAILWAY    MOTOR    AND    PERFORMANCE. 

then  to  throw  them  into  multiple,  so  that  each  is  subjected  to  the  full 
line  pressure  for  increased  speed. 

1388.  How  can  a  motor,  be  reversed  so  as  to  run  in  the  opposite 
direction? 

By  reversing  either  the  field  or  the  current  through  the  armature, 
the  pull  is  reversed  and  consequently  the  direction  of  rotation  of  the 
armature.  If  both  are  changed,  the  direction  of  rotation  is  the  same 
as  before.  For  this  reason,  it  makes  no  difference  which  terminal  of 
a  motor  is  positive.  The  relative  connections  between  the  armature 
and  field  must  be  reversed. 


CHAPTER    XII. 


ALTERNATING  CURRENTS. 

1400.  What  is  an  alternator? 

An  alternator  is  an  electrical  generator  that  delivers  an  alter- 
nating-current, as  distinguished  from  a  dynamo  giving  continuous 
or  direct  current. 

1401.  How  is  an  alternating  current  different  from  a  continuous 
or  direct  current? 

A  continuous  or  direct  current  is  of  uniform  strength  and  flows 
in  one  direction.  An  alternating  current  is  continually  changing 
both  its  strength  and  direction.  Alternating  currents,  such  as 
used  for  lighting  and  power,  follow  more  or  less  closely  what 
is  known  as  a  sine  curve.  The  current  begins  from  zero  value 
and  increases  rapidly  to  a  maximum  value  and  then  decreases  to 
zero,  then  increases  in  the  other  direction  to  a  maximum  value  and 
again  decreases  to  zero,  somewhat  as  indicated  in  the  figure  in  which 


FIG.  1401.— SINE  CURVE. 

horizontal  distances  represent  time  and  vertical  distances  represent 
the  varying  values  of  current  or  E.M.F.  -This  cycle  is  repeated  from 
25  to  130  or  more  times  per  second  with  the  machines  generally  used 
for  lighting  and  power.  (See  No.  1498.) 

1402.     For  what  purposes  are  alternating  currents  used? 
For  heat,  light  and  power  distribution,  also  for  medical  purposes. 
In  fact,  alternating  currents  are  as  suitable  as  continuous  for  almost 


338 


ELECTRICAL   CATECHISM. 


any  purpose  except  electro-plating1  and  other  chemical  work.     For 
arc  lights  the  alternating  current  is  not  ?lways  as  suitable  as  cor- 


No.  1  Power  House 
Ten  5000  -  H.P.  Generators 


B.   Botary  for  receiving  alternatiii 
and  delivering  direct  current 

V.    Begulator  for  changing  voltage 
A.C.   Alternating  current 
D.C.  Direct  aurrent 


BUFFALO  -  26  miles  from  Niagara 


D.C.  -  125  to  165  volts 
Rotaries 


__A.C.  - 100  volts 
6  "  _,  Transformers 


D.C  -  170  to  230  volts 

notaries 


Induction  Motors 
in  Factories 


Lighting  Factories 
and  Residences 


Distribution  circuits  supplied  by 
transformers 


D.C.  -  550  volts 


FIG.  1402.—  DIAGRAM  OF  CIRCUITS  FROM  NIAGARA  PLANT. 


tinuous  current.     The  diagram  shows  some  of  the  ways  in  which 
alternating  current  is  used  from  Niagara  Falls. 


ALTERNATING  CURRENTS. 


339 


1403.  Has  the  alternating  current  any  advantages  over  continu- 
ous currents? 

Very  heavy  currents,  such  as  used  for  welding,  or  currents  at 
very  high  voltages,  such  as  used  for  long-distance  transmission,  may 
be  obtained  more  easily  by  alternating  than  by  continuous  currents. 

1404.  How  are  very  heavy  currents  or  very  high  voltages  ob- 
tained? 

By  means  of  transformers,  sometimes  called  converters.  A  trans- 
former consists  of  a  laminated  iron  core  more  or  less  completely  sur- 
rounded by  two  coils  or  sets  of  coils,  one  consisting  of  comparatively 
few  turns  of  coarse  wire,  the  other  of  many  turns  of  fine  wire  as 


FIG.  1404.-SECTIONS  OF  TRANSFORMER. 

suggested  by  the  figure.  If  an  alternating  current  of  comparatively 
high  voltage  and  low  amperage  passes  through  the  fine  wire  coil,  a 
current  of  low  voltage  and  high  amperage  may  be  obtained  from  the 
other.  The  converse  is  also  true  (see  No.  1419). 

1405.     Explain  the  elementary  principles  of  the  transformer. 

The  fundamental  principle  of  the  transformer  is  practically  the 
same  as  that  of  a  dynamo  (see  Nos.  I  no  to  1114),  namely,  an 
E.M.F.  is  set  up  by  any  change  in  the  number  of  lines  of  force  en- 
closed by,  or  threading  through,  a  circuit.  The  alternating  current 
through  one  coil  causes  the  iron  core  to  be  magnetized,  demagnetized 
and  remagnetized  in  the  opposite  direction,  corresponding  to  every 
cycle  in  the  magnetizing  current.  This  changing  magnetization  in- 
duces corresponding  E.M.F's  in  every  coil  surrounding  the  iron  core, 
the  E.M.F.  in  each  coil  being  proportional  to  the  rate  of  change  of 
magnetization  and  also  to  the  number  of  turns  of  wire  in  the  coil. 
There  is  thus  an  induced  E.M.F.  in  the  magnetizing  coil  as  well  as  in 
the  other.  In  the  case  of  the  magnetizing  coil  the  induced  E.M.F. 
is  nearly  equal  to  the  E.M.F.  that  causes  the  magnetizing  current, 


340  ELECTRICAL   CATECHISM. 

and  is  almost  directly  opposed  to  it,  being  known  as  a  counter 
E.M.F.  or  C.E.M.F.  Since  the  induced  E.M.F.  is  proportional 
to  the  number  of  turns  of  wire  about  the  iron  cores,  it  follows  that 
the  E.M.F's  or  voltages  in  the  two  coils  have  the  same  ratios  as  the 
number  of  turns  in  their  respective  coils.  For  example,  if  one  coil 
has  ninety  turns,  while  the  other  has  only  nine,  the  voltage  in  the 
first  coil  is  ten  times  that  in  the  second.  By  similar  reasoning  it  may 
be  shown,  as  experience  proves,  that  the  current  bears  the  inverse  of 
the  same  ratio.  For  example,  if  one  coil  carries  I  amp.  at  100  volts, 
the  other  would  give  about  10  amps,  at  10  volts,  the  product  of 
amperes  by  volts  in  both  coils  being  nearly  equal.  As  a  matter  of 
fact  there  is  a  loss,  which  may  vary  from  I  to  as  high  as  15  per  cento 

1406.  In  a  transformer,  why  does  the  secondary  E.M.F.  bear  a 
definite  ratio  to  the  E.M.F.  in  the  primary  circuit? 

Assuming  that  the  losses  in  the  transformer  are  so  small  as  to  be 
negligible,  the  same  number  of  magnetic  lines  of  force  pass  through 
both  primary  and  secondary  coils.  Since  the  E.M.F's  in  the  two 
coils  are  proportional  to  the  number  of  lines  of  force  multiplied  by 
the  number  of  turns  in  the  coil,  it  follows  that  the  E.M.F's  are  di- 
rectly proportional  to  the  number  of  turns  in  the  two  coils, 

1407.  How  can  the  voltage  of  the  secondary  circuit  be  changed 
without  changing  the  primary  voltage? 

In  a  transformer  with  fixed  voltage  and  number  of  turns  in  the 
primary  circuit,  the  secondary  voltage  is  proportional  to  the  number 
of  turns  in  series  in  the  secondary  coil  (see  Nos.  1405  and  1406), 
and  to  the  effective  magnetic  flux.  The  secondary  voltage  may  be 
changed  by  varying  the  number  of  turns  in  series  in  the  secondary 
circuit  (see  Nos.  1410  and  1472),  or  by  varying  the  effective  flux 
through  the  secondary  (see  Nos.  1425  and  1426). 

1408.  Why  do  some  transformers  have  more  than  four  terminals? 
Manufacturers  commonly  wind  transformers  with  the  primary 

and  secondary  each  in  two  coils  or  sections.  The  two  primary  coils 
are  connected  in  series  when  intended  for  use  on  a  circuit  of  about 
2200  volts,  and  are  connected  in  multiple  when  intended  for  use  on 
circuits  of  about  i  TOO  volts.  In  order  to  avoid  danger  from  improper 
connections,  the  primary  coils  are  often  connected  inside  the  case  for 
i  loo  or  2200  volts,  as  may  be  ordered,  and  only  two  terminals  for 
the  primary  coils  come  through  the  outside  case.  The  four  terminals 
of  the  secondary  coils  commonly  come  outside  and  may  be  coupled 


ALTERNATING  CURRENTS. 


341 


in  series  or  multiple,  according  to  whether  the  secondary  pressure 
is  to  be  no  or  55  volts,  or,  in  later  practice,  220  or  no  volts. 


FIG.  1408,— SHELL-TYPE    TRANSFORMER. 

1409.  How  can  one  tell  which  terminals  are  primary  and  which 
are  secondary? 

The  wire  in  the  high-voltage  coils  is  always  smaller  than  that  in 
the  low-voltage  coils,  because  it  is  required  to  carry  less  current. 
Usually  the  wires  coming  through  the  outside  case  are  much 
larger  than  the  wires  in  the  coils,  because  of  their  greater  liability 
to  damage  in  handling.  Thus  the  outside  wires  are  not  the  same 
size  as  those  inside,  but,  as  a  rule,  larger  wires  are  used  for  the  low- 
voltage  coils  than  for  the  others. 

1410.  How  can  one  tell  which  terminals  of  the  secondary  coils  are 
io  be  connected  together  in  order  to  place  the  coils  in  series? 

The  manufacturers   generally   send   a   diagram  with  the  trans- 


342 


ELECTRICAL  CATECHISM. 


former,  such  as  that  illustrated,  showing  which  terminals  are  to  be 
connected.  If  no  diagram  is  available,  connect  the  primary  terminals 
to  the  line  and  find  out  which  terminals  go  with  each  coil.  This  can 
be  done  by  connecting  one  side  of  an  incandescent  lamp  to  one  ter- 
minal of  one  coil  and  then  touching  the  other  lamp  terminal  to  one 
wire  after  the  other  until  one  is  found  that  will  make  the  lamp  light 
up ;  this  wire  is  the  other  terminal  of  the  same  coil.  In  the  same  way 
test  the  other  two  terminals  to  see  if  the  lamp  will  light  up  when 
connected  between  them.  Now  connect  one  terminal  of  one  coil  to 
one  terminal  of  the  other  and  connect  the  lamp  between  the  other 
two  terminals.  If  it  comes  up  to  brightness  it  shows  that  the  coils 
are  in  series  so  that  their  E.M.F's  are  added.  If  the  coils  are  work- 
ing against  one  another,  their  voltages  will  balance  and  the  lamp  will 
not  light.  See  also  Fig.  3376. 


"PRIMARY  CONNECTED  FOR  1050  VOLTS.    PRIMARY  CONNECTED  FOR  2100  VOLTS. 


SECONDARY  CONNECTED 
FOR  105  VOLTS. 


SECONDARY  CONNECTED 
FOR  210  AND  420  VOLTS. 


FIG.  1410.— TRANSFORMER   CONNECTION    DIAGRAM. 

1411.  How  can  one  tell  the  proper  way  to  connect  the  secondary 
coils  of  a  transformer  in  multiple? 

Find  the  terminals  of  each  coil  as  shown  above  (No.  1410) 
and  connect  one  terminal  of  one  coil  to  one  terminal  of  the  other. 
Connect  the  lamp  between  the  outer  terminals.  If  the  lamp  does  not 
light  up  with  such  connection,  while  it  does  light  up  when  connected 
across  either  coil  alone,  the  outer  terminals  may  be  connected  safely. 


ALTERNATING  CURRENTS. 


343 


1412.  Is  there  any  harm  in  improperly  connecting  the  coils  in 
multiple? 

If  one  coil  is  connected  "upside  down"  to  the  other  coil,  the  two 
are  on  a  short-circuit  with  both  E.M.F's  in  the  same  direction  send- 
ing current  through  the  coils.  The  result  is  that  both  coils  will  be 
burnt  out  if  the  current  in  the  primary  coil  is  not  immediately  cut 
off  by  the  fuse  or  otherwise. 

1413.  Can  the  secondaries  of  two  transformers  be  safely  con- 
nected in  series? 

There  is  no  danger  in  such  connection.  Care  must  be  taken  to  con- 
nect the  right  terminals  so  that  the  E.M.F's  of  the  two  secondaries 
assist  rather  than  neutralize  one  another. 

1414.  Can  the  secondaries  of  two  transformers  be  safely  con- 
nected in  multiple ? 

Care  must  be  taken  to  get  them  properly  connected,  just  as  in  the 
case  of  coupling  the  two  secondary  coils  of  a  single  transformer,  as 


FIG.  1414.— LARGE    OIL-COOLED   TRANSFORMERS    IN    MULTIPLE. 

considered  in  No.  1412.  The  E.M.F's  may  be  tested  in  the  same 
manner  as  there  outlined.  Care  must  be  taken  that  the  secondaries 
give  equal  E.M.F's. 


344 


ELECTRICAL   CATECHISM. 


1415.  Will  the  load  be  properly  divided  if  two  transformers  have 
their  secondaries  connected  in  multiple? 

It  will  if  the  two  transformers  have  similar  regulation ;  that  is,  if 
the  voltage  of  each  drops  oft"  the  same  number  of  volts  at  half  load 
or  at  full  load. 

1416.  For  ivhat  changes  of  voltage  are  transformers  commonly 
made? 

The  standard  practice  is  to  wind  transformers  for  a  ratio  of  some 
multiple  of  five.  Since  both  high  and  low  tension  windings  are 
usually  wound  in  several  sections,  they  may  be  connected  in  series  or 


FIG.  1416.— OIL-COOLED   TRANSFORMER   WITH    CASE   REMOVED. 


in  multiple  to  give  two  or  more  ratios  (see  Fig.  1410).  With  series 
connection,  "  taps  "  are  sometimes  brought  out  from  intermediate 
points,  so  that  two  or  more  voltages  may  be  had  simultaneously. 
For  general  distribution  it  is  common  to  have  the  secondaries  give 
no  or  220  volts,  the  primaries  being  supplied  noo,  2200  or  4400 


ALTERNATING  CURRENTS.  345 

volts.  For  transmission  lines  the  generators  are  wound  for  noo  to 
6600  volts,  the  transformers  stepping  up  to  from  6600  to  88,000 
volts,  depending  on  the  distance.  These  are  nominal  voltages,  since 
the  generator  pressure  must  be  sufficiently  high  to  care  for  the  vari- 
ous drops  in  the  lines  and  transformers. 

1417.  What  arc  the  sizes  of  ordinary  transformers? 

They  range  in  size  from  instrument  transformers  about  6  inches 
square,  taking  25  to  50  watts,  to  the  7500  kilowatt  transformers  at 
Duluth  which  are  14  feet  10  inches  high,  14  feet  long  and  5  feet  10 
inches  thick  and  weighing  171,000  pounds.  Ordinary  transformers 
for  light  and  power  run  from  0.6  to  2000  kilowatts  capacity. 

1418.  What   is   meant   by    "step-up"  and   "step-doivn"    trans- 
formers? 

A  step-up  transformer  is  one  used  for  changing  the  voltage  from 
low  to  high.  A  step-down  transformer  changes  the  voltage  from 
high  to  low.  There  is  no  difference  between  the  two  transformers 
except  in  their  use. 

1419.  How  can  the  same  transformer  be  used  either  to  raise  or  to 
lower  the  secondary  voltage? 

The  ratio  between  the  voltages  of  the  two  coils  depends  upon  the 
ratio  between  the  number  of  turns  of  wire  on  the  two  coils.  Since 
both  coils  surround  the  same  core  it  follows  from  the  elementary 
principles  laid  down  before  (see  No.  1405)  that  if  either  coil  is  con- 
nected to  a  suitable  source  of  supply  it  will  magnetize  the  core  and 
induce  E.M.F.  in  the  other  coil.  The  secondary  voltage  will  be 
higher  or  lower  than  the  primary,  according  to  the  ratio  of  the  turns 
of  wire  in  the  two  coils. 

1420.  What  prevents  an  enormous  current  from  passing  through 
the  primary  coil  and  burning  it  out? 

The  current  is  opposed  by  the  C.E.M.F.  mentioned  above  in  No. 
1405.  The  small  alternating  current  that  passes  through  the  pri- 
mary coil  magnetizes  the  iron  core,  first  in  one  direction  and  then 
in  the  other.  This  rapid  magnetization  and  demagnetization  means 
that  the  number  of  magnetic  lines  of  force  threading  through  the 
iron  core  inside  of  the  coils  is  continually  changing.  Looked  at  from 
a  slightly  different  standpoint,  it  means  that  lines  of  magnetic  force 
are  continually  crossing  the  coils,  or  that  the  coils  are  continually 
cutting  the  lines  of  force.  The  result  is  the  same  viewed  from  either 
standpoint,  the  changing  magnetization  of  the  iron  core  causing 
E.M.F's  in  the  coils  surrounding  the  iron.  The  E.M.F's  thus  in- 
duced in  the  secondary  coil  cause  current  to  flow  in  the  secondary  cir- 


346  ELECTRICAL   CATECHISM. 

cuit.  Likewise,  the  E.M.F's  similarly  induced  in  the  primary  coil 
tend  to  send  current  through  the  primary  circuit  in  opposition  to  the 
original  current.  Thus,  the  E.M.F.  induced  in  the  primary  coil  acts 
as  a  C.E.M.F.  opposing  the  impressed  E.M.F.  or  voltage  on  the 
primary  circuit,  and  so  holding  back  the  current. 

1421.  Does  the  primary  coil  of  a  transformer  always  take  the 
same  current,  or  does  the  current  vary  according  to  the  current  taken 
from  the  secondary? 

The  current  through  the  primary  is  almost  exactly  proportional 
to  that  through  the  secondary.  For  example,  a  5o-light  or  25oo-watt 
transformer  for  reducing  from  1000  to  100  volts  takes  about  one- 
fourth  of  an  ampere  (0.23)  when  the  secondary  circuit  is  open;  this 
increases  to  about  3  amps,  when  the  secondary  circuit  supplies  forty 
lamps. 

1422.  Does  the  resistance  of  the  primary  circuit  change,  so  as  to 
let  more  current  pass  when  the  secondary  gives  more  current? 

No.  The  resistance  of  the  primary  circuit  remains  constant. 
Ohm's  law,  in  its  simple  form,  does  not  always  hold  true  for  alter- 
nating currents.  For  instance,  the  primary  circuit  of  the  trans- 
former just  mentioned  has  a  resistance  of  not  quite  3  ohms.  One 
might  expect  that  when  such  a  resistance  was  connected  across  a 
looo-volt  circuit,  more  than  300  amps,  would  pass,  unless  something 
happened  to  open  the  circuit.  As  a  matter  of  fact,  less  t'nan 
one-fourth  of  I  amp.  flows  through  the  primary  of  this  transformer, 
if  the  secondary  circuit  is  open  so  that  it  delivers  no  current. 

1423.  How  does  the  current  taken  by  the  primary  increase  when 
the  secondary  current  increases? 

This  matter  is  somewhat  complicated,  but  may  be  explained  in  an 
elementary,  although  incomplete,  way  as  follows :  Since  the  in- 
duced E.M.F's  in  the  coils  are  in  the  opposite  direction  to  the  E.M.F. 
sending  current  through  the  primary  coil,  it  follows  that  the  current 
in  the  secondary  circuit  is  also  in  the  opposite  direction  to  that  in  the 
primary.  The  result  is  that  the  secondary  current  tends  to  mag- 
netize the  iron  core  in  a  direction  opposite  that  of  the  primary  cur- 
rent. This  opposing  magnetizing  force  would  weaken  the  re- 
sultant magnetization  and  so  reduce  the  E.M.F's  in  the  two  coils. 
But  the  reduction  in  the  C.E.M.F.  in  the  primary  coil  allows  more 
current  to  pass  through  the  primary  coil  and  so  to  bring  up  the  volt- 
ages in  both  coils  to  nearly  the  same  values  they  had  before.  In 
practice  it  is  found  that  this  complex  action  goes  on  regularly,  so 
that  the  current  through  the  primary  increases  in  almost  exact  pro' 


ALTERNATING  CURRENTS.  347 

portion  to  that  in  the  secondary.  The  more  complete  theory  of  this 
reaction  is  complicated  by  the  fact  that  the  E.M.F's  and  currents  in 
the  two  circuits  reach  their  corresponding  maximum  values  at  dif- 
ferent times,  the  primary  current  always  being  more  or  less  behind 
the  primary  E.M.F. 

1424.  How  can  this  be  explained  more  briefly  f 

When  current  is  taken  from  the  secondary,  this  current  sets  up 
lines  of  force  opposing  those  of  the  primary,  and  the  lines  thus 
"killed"  are  supplied  by  more  current  flowing  into  the  primary. 
Aside  from  this  explanation,  from  the  law  of  conservation  of  energy, 
the  primary  energy  must  equal  the  secondary  energy  (leaving  aside 
losses)  ;  and  since  the  two  voltages  are  assumed  to  remain  constant, 
the  primary  and  secondary  currents  must  increase  or  decrease  to- 
gether. 

1425.  What  are  series  transformers? 

Series  transformers,  sometimes  called  "  current  transformers  "  or 
"  safety  coils,"  are  used  on  high  potential  circuits  for  obtaining  cur- 
rent proportional  to  the  main  current  but  at  low  potential.  They 
are  made  for  current  ratios  of  from  i  :i  to  i  1200.  At  switchboards 
they  are  used  for  insulating  ammeters,  current  coils  of  wattmeters 
and  for  relay  coils  of  automatic  circuit  breakers  (see  No.  422).  In 
composite  alternators  they  are  used  for  obtaining  commutating  cur- 
rent proportional  to  that  in  one  or  all  of  the  phases  (see  Nos.  1468- 
1470).  They  are  used  for  operating  10  or  12  ampere  flaming  arcs 
from  a  4,  6.6  or  7.5  ampere  circuit  (see  No.  485).  They  are  some- 
times used  for  operating  indoor  series  lamps  without  carrying  high 
tension  lines  into  buildings.  Care  should  be  taken  not  to  open  the 
secondary  of  a  constant-current  transformer,  as  the  pressure  may  rise 
dangerously  high  because  of  the  increase  in  magnetic  flux  due  to  the 
absence  of  the  countermagnetizing  secondary  current  (see  Nos.  1405, 
1423).  The  safe  plan  is  to  short-circuit  the  secondary,  in  case  it  is 
necessary  to  make  changes  while  power  is  on.  The  current  trans- 
former is  based  on  the  same  principles  as  the  constant  potential  trans- 
former, differing  only  in  details  of  design  and  method  of  use.  (  See 
also  No.  1472.) 

1426.  Can  transformers  be  made  to  give  secondary  current  of 
constant  strength  when  the  primary  is  connected  to  a  constant- 
potential  circuit? 

Several  companies  make  transformers  that  give  approximately 
constant  current  within  wide  ranges  of  voltage  when  the  primary 


348 


ELECTRICAL   CATECHISM. 


is  supplied  at  constant  potential.  This  is  usually  accomplished  by 
designing  the  transformer  so  that  there  will  be  considerable  magnetic 
leakage.  As  stated  before  (No.  1423),  the  current  in  the  secondary 


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FIG.  1426A.— OPERATION  OF  CONSTANT-CURRENT  TRANSFORMER. 

coil  has  a  magnetizing  effect  in  the  opposite  direction  to  that  of  the 
primary  current.  This  counter  magnetomotive  force  makes  it  more 
difficult  for  the  magnetic  field  from  the  primary  to  pass  through  the 
secondary, .  and  so  more  of  the  magnetic  lines  of  force  leak  across 
without  threading  through  the  secondary  coil,  as  suggested  in  the 
figure.  Since  the  E.M.F.  in  the  secondary  coil  is  proportional  to  the 
number  of  magnetic  lines  of  force  enclosed  by  it,  it  follows  that  the 
greater  the  amount  of  leakage  the  less  will  be  the  secondary  E.M.F. 
It  follows  further  that  if  the  secondary  current  increases,  the  leak- 
age will  also  increase,  thereby  diminishing  the  secondary  E.M.F.  and 
in  turn  diminishing  the  secondary  current.  By  suitably  designing 
the  magnetic  circuit,  the  leakage  field  will  increase  in  such  proportion 
to  the  increase  of  secondary  current  that  the  secondary  current  will 
remain  nearly  constant.  In  a  constant-current  transformer  of  the 
General  Electric  Company  the  amount  of  magnetic  leakage  is  varied 
by  an  automatic  adjustment  of  the  relative  positions  of  the  primary 
and  secondary  coils.  Several  manufacturers  obtain  nearly  constant 


ALTERNATING  CURRENTS. 


349 


FIG.    1426B.-CONSTANT-CURRENT,   ARC   LIGHT   TRANSFORMER. 

current  from  constant-potential  circuits  by  inserting  a  choke  coil 
with  variable  impedance.  (See  Nos.  1473  to  1477.) 

1427.  How  do  alternators  differ  from  direct-current  generators? 
Excepting  "unipolar"  machines,  which  have  not  proved  commercially 

successful,  all  direct-current  machines  (dynamos  or  motors)  require 
commutators  for  rectifying  the  current  (Nos.  1114-1117)  ;  the  need 
of  adjusting  the  brushes  requires  the  armature  to  be  the  moving 
part.  The  alternator  has  more  field  poles,  and  either  the  armature 
or  field  may  revolve  (see  1459-1461).  Ordinarily,  current  is  con- 
ducted to  and  from  the  rotor  through  brushes  bearing  on  continuous 
rings  called  collecting  rings  or  slip  rings.  (See  1428  and  1431.) 

1428.  What  is  meant  by  rotor  and  stator? 

Since  an  alternating-current  generator  or  motor  may  have  either 
the  armature  or  the  field  rotate,  the  revolving  part  is  commonly  called 
the  rotor,  and  the  stationary  part  the  stator. 


350 


ELECTRICAL   CATECHISM. 


FIG.  1426B.-CONSTANT-CURRENT,  ARC  LIGHT  TRANSFORMER. 


1429.  Would  a  direct-current  dynamo  give  an  alternating  current 
if  collecting  rings  were  substituted  for  the  commutator? 

It  would  if  means  were  provided  for  keeping  up  the  magnetic  field. 
But  this  would  not  be  the  most  economical  winding  for  an  alternator, 
and  a  machine  built  for  alternating  currents  would  be  wound  dif- 
ferently from  a  direct-current  machine.  Machines  are  made  having 
a  commutator  and  also  a  set  of  collecting  rings.  When  these  ma- 
chines are  driven  from  an  outside  source  of  power  they  are  called 
"double-current  generators."  The  same  machine  may  be  used  to 
change  direct  current  into  alternating  current,  or  vice  versa,  one 
kind  of  current  entering  at  one  set  of  terminals  and  the  other  kind  of 


ALTERNATING  CURRENTS.  351 

current  leaving  the  other  terminals.  In  the  latter  case  it  is  called  a 
"  rotary  converter,"  "rotary"  or  "  synchronous  converter."  (See 
Nos.  1468  to  1471  and  1510  to  1519.) 

1430.  For  what  purposes  are  double-current  generators  used? 
They  are  used  in  central  stations  which  deliver  alternating  -current 

to  some  feeders  and  direct  current  to  others,  the  same  machine  sup- 
plying two  different  services,  according  to  requirements,  thus  re- 
ducing the  number  and  capacity  of  machines  otherwise  necessary. 

1431.  How  does  the  winding  of  an  alternator  armature  differ 
from  that  of  a  direct-current  machine? 

In  modern  direct-current  machines,  all  the  coils  on  an  armature 
are  exact  duplicates.  Alternating-current  generators  have  two  gen- 
eral styles  of  winding,  either  all  coils  alike  or  else  several  forms 
of  coil  in  the  same  machine.  By  varying  the  spacing  and  connec- 
tions of  the  coils,  all  of  their  E.M.F's  may  be  combined  into  one  so 
as  to  give  a  single-phase  current,  or  so  as  to  give  two-phase  or 
three-phase  currents  (see  Nos.  1437  to  1451  and  1459  to  1461). 
When  each  circuit  has  one  coil  per  pole,  the  winding  is  said  to  be 
"  uni-tooth ;"  when  there  are  two  or  more  coils  per  pole  per  phase, 


FIG.  1431.— ALTERNATOR    WITH     REVOLVING     FIELD    AND     WITH     GROUP 

WINDINGS. 


the  winding  is  said  to  be  "  multi-tooth,"  "  poly-odontal "  or  "  distrib- 
uted," such  distribution  lowering  the  total  E.M.F.  somewhat,  but 
lessening  the  armature  magnetic  disturbance  and  tending  toward 
giving  a  more  nearly  sinusoidal  E.M.F.  Alternators  with  only  one 
coil  per  pole  per  phase,  especially  single-phase  machines,  are  apt  to 


352  ELECTRICAL  CATECHISM. 

be  noisy  and  to  give  trouble  from  heating  in  the  pole-  tips  on  account 
of  the  surging  of  the  magnetic  field  as  the  wide  teeth  and  slots  pass. 
Most  modern  alternators  have  revolving  fields. 

1432.  How  many  poles  do  alternators  have? 

Small  magneto  machines  for  telephone  signals  and  testing  have 
only  two  poles.  Alternators  driven  by  steam  turbines,  which  run 
at  high  speed,  have  two  to  eight  poles.  Other  alternators  have  from 
six  to  sixty  or  more  poles.  (See  No.  1121.) 

1433.  Why  do  alternators  have  so  many  poles? 

In  order  to  get  high  frequency  without  excessive  speed  of  arma- 
ture. The  alternating  currents  used  practically  make  from  3000  to 
16,800  reversals  per  minute,  or  make  from  25  to  140  complete  cycles 
per  second,  the  more  common  modern  practice  in  America  being  25 
and  60  cycles  per  second,  European  practice  favoring  25  and  50  cycles. 

1434.  What  limits  the  frequency  of  alternating  currents? 

The  lower  limit  is  governed  by  the  use  made  of  the  current.  Un- 
less the  frequency  is  as  high  as  about  40  cycles  per  second,  arc  lamps 
will  not  burn  well  and  incandescent  lights  are  apt  to  flicker.  High 
frequencies  were  used  when  alternating  currents  were  first  intro- 
duced in  order  to  allow  transformers  to  be  made  small  and  cheap 
for  a  given  output  to  compete  with  low-voltage  direct-current  sys- 
tems. But  with  the  high  frequency  it  is  more  difficult  to  operate  mo- 
tors satisfactorily,  and  the  difficulties  from  induction  on  the  circuits 
increase.  Modern  practice  favors  50  and  60  cycles  for  general  use 
and  25  for  power  only.  For  certain  experimental  work,  frequen- 
cies running  up  into  the  thousands  per  second  have  been  used,  but 
these  have  not  yet  reached  the  commercial  stage. 

1435.  How  can  the  frequency  of  an  alternating  current  be  meas- 
ured? 

The  frequency  equals  the  number  of  pairs  of  poles  in  the  alter- 
nator multiplied  by  the  number  of  revolutions  per  second.  For  ex- 
ample, a  turbo-generator  having  eight  poles  and  running  at  15  revo- 
lutions per  second  gives  a  frequency  of  60  cycles  per  second. 

1436.  How  can  the  number  of  alternations  be  determined? 
The  number  of  alternations  per  minute  equals  the  number  of  poles 

multiplied  by  the  number  of  revolutions  per  minute.  For  example, 
the  machine  with  ten  poles  running  at  1500  r.  p.  m.  gives  15,000 
alternations  per  minute. 


ALTERNATING  CURRENTS.  353 

1437.     What  is  a  tzvo-phase  current? 

By  a  two-phase  current  is  meant  two  separate  alternating  currents 
which  are  maintained  at  a  constant  difference  of  phase.  These  cur- 
rents are  90  degrees  apart,  one  current  being  at  its  maximum  value 
when  the  other  is  at  its  zero  value.  This  is  illustrated  in  the  figure, 
in  which  the  solid  curve,  B,  represents  one  current  and  the  dotted 


FIG.  1437.— CURRENTS  90  DEGREES  APART. 

curve,  A,  the  other,  the  value  of  the  current  at  any  instant  being 
represented  by  its  distance  above  or  below  the  straight  horizontal 
line,  the  distance  from  o  to  o"  representing  the  time  required  for  a 
complete  cycle.  Each  current  is  usually  carried  on  its  own  circuit  in- 
dependent of  the  other.  "Quarter-phase"  is  another  name  for  two-phase. 

1438.  How  are  two-phase  currents  obtained? 

One  way  is  by  the  use  of  four  collecting  rings  connected  with  four 
points  on  a  direct-current  commutator,  one  pair  being  connected  to 
points  directly  under  brushes  of  opposite  polarity,  the  other  pair 
being  connected  to  points  midway  between  these.  Another  method  is 
to  couple  the  armature  shafts  of  two  similar  alternator  armatures 
together  so  that  the  E.M.F.  of  one  is  a  maximum  at  the  same  instant 
that  the  E.M.F.  of  the  other  is  at  zero.  The  more  usual  method  for 
commercial  work  is  to  use  a  regular  two-phase  alternator. 

1439.  What  is  a  two-phase  alternator? 

A  two-phase  alternator  has  two  separate  windings  which  are  so 


FIG.   1439.-ELEMENTARY  TWO-PHASE   ALTERNATOR. 

arranged  that  the  E.M.F.  in  one  is  zero  at  the  instant  when  the 
E.M.F.  of  the  other  is  at  a  maximum.    This  is  illustrated  diagram- 


354 


ELECTRICAL   CATECHISM. 


matically  in  the  figure,  which  represents  a  two-pole  machine  with 
two  coils  at  right  angles.  In  the  sketch  shown,  the  coil,  A, 
is  at  the  position  of  zero  E.M.F.,  while  coil  B  is  in  the  position  of 
greatest  E.M.F.  In  the  actual  two-phase  alternators  there  are,  of 
course,  more  poles  and  more  coils  in  each  circuit,  the  two  sets  being 
arranged  so  that  the  coils  of  one  set  are  between  the  poles  when  the 
others  are  under  them. 

1440.     What  is  a  three-phase  current? 

A  so-called  three-phase  current  consists  of  three  separate  currents 
in  three  separate  circuits,  the  three  being  120  electrical  degrees  apart, 


FIG.    1440.-CURRENTS   120   DEGREES    APART. 

as  illustrated  in  the  figure.  If  the  time  or  distance  from  o  to  o"  be 
considered  as  divided  into  360  degrees,  representing  an  entire  circle 
or  cycle,  corresponding  values  of  the  three  currents  are  120  degrees 
apart.  See  also  No.  1453. 

1441.     How  are  three-phase  currents  obtained? 

By  collecting  rings  coupled  to  points  on  a  direct-current  generator 
1 20  degrees  apart,  considering  the  distance  from  a  positive  to  a  nega- 
tive brush  on  the  commutator  as  being  180  degrees  or  one-half  cycle. 


FIG.   1441.— ELEMENTARY   THREE-PHASE   ALTERNATOR. 

Also  three  single-phase  alternator  armatures  may  have  their  shafts 
mechanically  coupled.  Regular  three-phase  alternators  have  three 
distinct  armature  windings  equally  spaced ;  in  the  actual  machine, 
these  are  connected  at  one  or  three  points  (see  Nos.  1450  and  1451), 
so  as  to  require  only  three  collecting  rings  or  terminals. 


ALTERNATING  CURRENTS. 


355 


1442.  What  are  polyphase  currents,  and  for  what  are  they  used? 
Polyphase  and  multiphase  are  names  for  two  or  more  alternating 

currents  having  a  fixed  phase  difference,  such  as  two-phase  (quarter- 
phase),  three-phase  or  six-phase.  They  are  more  satisfactory  for 
operating  motors,  except  small  ones;  power  is  transmitted  cheaper 
by  three-phase  than  by  single-phase  or  by  direct  current. 

1443.  Is  it  necessary  to  have  four  wires  for  carrying  two-phase 
currents? 

Four  wires  are  generally  used,  two  for  each  circuit,  as  suggested 
in  the  figure,  in  which,  for  simplicity,  the  transformers  are  omitted. 


ALJERNATOH 


FIG.  1443A.-FOUR-WIRE  TWO-PHASE  SYSTEM. 


ALTERNATOR  LINE  LOAD 

FIG.  1443B.-THREE-WIRE  TWO-PHASE  SYSTEM. 

Sometimes  one  common  wire  and  two  outers  are  used,  as  in  the  direct- 
current  three-wire  system,  except  that  in  the  two-phase  system  the 
common  wire  should  be  1.4  larger  than  the  outers. 

i/|/|/|.  In  the  three-wire  two-phase  system,  is  the  common  wire  the 
same  as  the  others? 

In  order  to  give  the  same  loss  on  the  line,  the  common  wire  should 
be  1.414  times  larger  than  the  other  wires,  because  the  current  in  the 
common  wire  is  the  resultant  of  the  two  currents  in  the  other  wires. 

1445.  Is  not  the  current  in  the  common  wire  equal  to  the  sum  of 
the  two  currents ? 

Yes  and  no.  At  any  instant  the  current  in  the  common  wire  equals 
the  sum  of  the  two  currents.  But  an  ammeter  in  the  common  wire 
would  not  indicate  as  much  as  the  sum  of  the  ammeter  readings  in  the 
two  circuits.  For  instance,  if  each  circuit  was  carrying  10  amperes, 
the  common  wire  would  be  carrying  14.14  amps. 


356 


ELECTRICAL   CATECHISM. 


1446.  Do  not  the  rules  of  ordinary  arithmetic  apply  to  alternating 
currents? 

They  do  when  rightly  applied.  Ordinarily,  10  -f-  10  =  20,  but  in 
this  case,  10  +  10  —  14.14.  The  same  thing  is  true  in  many  other 
cases  with  alternating  currents.  The  difficulty  is  caused  by  the  fact 
that  the  two  currents  or  E.M.F's  may  have  a  difference  of  phase,  so 
that  they  do  not  reach  corresponding  values  at  the  same  time.  At 
any  particular  instant,  the  current  in  the  common  wire  is  the  sum  of 
the  two  currents  at  that  same  instant.  But  the  ammeter  does  not 
measure  the  instantaneous  values  of  the  current,  it  measures  the 
effective  current.  If  the  two  currents  were  in  the  same  phase,  reach- 
ing corresponding  maximum  values  at  the  same  time,  the  resultant 
current  in  the  common  wire,  as  measured  by  the  ammeter,  would  be 
equal  to  the  sum  of  the  two  currents,  and,  in  the  above  example, 
the  ammeter  would  show  20  amps,  in  the  common  wire.  On  the  other 
hand,  if  the  two  currents  were  180  degrees  apart,  one  reaching  its 
maximum  positive  value  at  the  instant  when  the  other  reached  its 
maximum  negative  value,  the  two  currents  in  the  common  wire 
would  exactly  neutralize  one  another,  and  the  ammeter  would  stand 
at  zero.  In  this  case  the  two  circuits  would  be  practically  in  series, 
the  current  being  the  same  on  each  side,  and  the  voltage  between  the 
two  outside  wires  being  double  that  between  either  outside  wire  and 
the  neutral  wire. 

1447.  What  is  the  sum  of  two  alternating  currents  po  degrees 
apart? 

The  way  in  which  the  two  currents  are  combined  in  the  common 
wire  to  form  a  resultant  less  than  their  sum  is  illustrated  in  the 


FIG.  1447.— COMPOSITION  OF  TWO  CURRENTS. 

figure.  Suppose  that  the  sine  curve,  A,  represents  the  current  com- 
ing into  the  common  wire  from  one  circuit,  while  B  represents  the 
other  current.  The  value  of  the  resultant  current,  R,  is  found  at 


ALTERNATING  CURRENTS. 


357 


any  instant  by  adding  the  values  of  A  and  B  at  that  instant.  For 
example,  at  the  instant  marked  45  degrees  the  value  of  B  is  negative 
and  exactly  equal  to  the  value  of  A  at  the  same  instant,  so  that  the 
instantaneous  sum  or  the  resultant  of  the  two  is  zero.  Again  at  the 
point  marked  90  degrees,  B  is  at  zero,  while  A  is  at  its  positive  maxi- 
mum value,  and  the  sum  equals  A.  A  little  later,  at  the  point  marked 
135  degrees,  both  B  and  A  are  positive  and  the  resultant  equals  their 
sum  at  that  instant.  By  following  this  analysis  through  the  cycle,  it 
is  seen  that  the  curve  representing  the  resultant  or  sum  of  the  simul- 
taneous values  is  a  curve  similar  to  the  others,  and  its  mean  value 
bears  the  same  ratio  to  its  maximum  value  that  the  mean  value  of 
either  of  the  component  curves  does  to  its  maximum. 

1448.     How  are  two-phase  alternator  armatures  wound? 
The  two  circuits  may  be  entirely  distinct,  each  having  two  collect- 
ing rings  as  suggested  in  Fig.  1443.     Or  the  two  circuits  may  be 


FIG.  1448.-ELEMENTARY  TWO-PHASE  ARMATURES. 

coupled  at  a  common  middle  point  as  suggested  in  the  accompanying- 
figure,  each  circuit  having  two  collecting  rings.  Or  the  two  circuits 
may  be  coupled  in  the  armature  so  that  only  three  collecting  rings 
are  required,  as  suggested  in  the  third  figure. 


FIG.  1449.-ELEMENTARY  THREE-PHASE  ALTERNATOR. 

1449.     How  are  three-phase  alternator  armatures  wound? 
There  are  generally  three  coils  or  sets  of  coils,  120  electrical  de- 
grees apart,  as  in  the  ideal  sketch  in  the  figure.    Instead  of  having" 


358 


ELECTRICAL   CATECHISM. 


two  terminals  for  each  coil  or  circuit,  these  are  generally  connected 
so  that  only  three  or  four  collecting  rings  are  required. 

1450.     What  is  meant  by  a  mesh-connected  three-phase  armature ? 

When  connected  so  that  the  three  coils  form  a  complete  circuit 

within  themselves,  as  Fig.  1450  indicates,  it  is  said  to  be  a  mesh  or 


FIG.   1450.— "DELTA"   CONNECTED   ARMATURES. 

triangle  winding.    Sometimes  this  is  called  a  "delta"  connection,  be- 
cause the  Greek  letter  Delta  has  the  general  shape  of  a  triangle. 

1451.     What  is  meant  by  a  star-connected  three-phase  armature ? 

When  the  three  coils  are  connected  at  a  common  central  point,  as 

Fig.  1451  indicates,  it  is  called  a  star  or  Y  winding.      Three-phase 


.1 — i 


FIG.   1451.— "Y"   CONNECTED   ARMATURE. 


alternators  usually  have  three  collecting  rings,  although  there  may 
be  a  fourth  ring  connecting  with  the  common  center  of  the  star  wind- 
ing. 

1452.  Hoiv  many  wires  are  required  for  carrying  three-phase  cur- 
rents? 

Three  wires  are  generally  used  for  transmission  lines.  For  distrib- 
uting circuits,  the  armature  windings  or  the  transformer  secondaries 
are  frequently  connected  in  "  Y  "  or  star  fashion,  four  line  wires 
connecting  with  the  outer  terminals  and  with  the  center  or  neutral 
point.  Lamps  are  then  connected  between  the  neutral  and  an  outer. 
Four-wire  systems  are  less  disturbed  by  unequally  balanced  loads. 


ALTERNATING  CURRENTS.  359 

1453.  Hoiv  can  three  separate  currents  be  carried  on  three  wires 
and  yet  be  distinct? 

Each  wire  practically  becomes  the  return  wire  for  the  other  two 
circuits.  When  all  three  currents  are  equa1.  they  may  be  considered 
as  neutralizing  one  another  at  the  common  meeting  point  so  that  no 
return  wire  is  necessary.  This  may  be  understood  by  examining  the 


FIG.  1453.— THREE-PHASE  CIRCUITS. 

figure,  in  which  A,  B  and  C  represent  the  three  currents  120  degrees 
apart.  It  may  be  seen  that  at  any  instant,  for  example,  at  the  point 
marked  60  degrees,  the  negative  value  of  C  just  equals  the  sum  of 
the  positive  values  of  A  and  B.  In  the  same  way,  at  any  other  in- 
stant the  sum  of  three  currents  is  zero,  and  therefore  no  return  wire 
is  necessary. 

1454.  What  sort  of  current  is  used  for  the  field  coils  of  alter- 
nating-current machines? 

The  fields  are  excited  by  direct  current,  except  in  the  case  of  in- 
duction (not  inductor)  generators  and  of  induction  motors. 

1455.  Why  is  it  necessary  to  use  direct  current  in  the  field  coils 
of  alternators? 

The  machine  will  refuse  to  generate  E.M.F.  unless  the  field  is 
continuously  excited.  If  an  alternating  current  should  be  sent 
through  the  field  coils,  the  magnetic  field  would  follow  the  variations 
of  the  current,  a  given  pole  being  a  strong  north  pole  at  one  instant, 
then  weakening  to  zero  and  becoming  a  south  pole.  Since  a  mag- 
netic field  always  tends  to  oppose  changes  rather  than  to  make  them, 
the  field  would  have  no  tendency  to  reverse  of  itself,  and  therefore  the 
machine  would  not  pick  up.  It  would  be  like  trying  to  make  a  direct 
current  dynamo  pick  up  when  the  field  connections  were  reversed. 

1456.  How  is  the  exciting  current  obtained? 

In  some  early  alternators  a  special  set  of  coils  was  wound  on  the 
alternator  armature  and  connected  with  a  commutator  from  which 
a  direct  current  was  obtained  for  the  field  coils.  On  account  of  the 
difficulty  of  insulating  and  repairing  this  special  winding,  which 
was  usually  placed  under  the  alternating-current  armature  coils,  this 
method  was  abandoned.  Practically  all  the  alternators  in  use  at 


360 


ELECTRICAL   CATECHISM. 


present  take  their  field-exciting  current  from  a  small  direct-current 
machine  called  an  exciter. 

1457.     How  is  the  exciter  generally  driven? 

The  exciter  is  a  direct-current  generator  driven  from  the  alternator 
shaft  or  from  some  other  source  of  power.     Frequently  it  is  mounted 


FIG.    1457.-ALTERNATOR   AND    BELTED    EXCITER. 

on  the  alternator  frame,  being  driven  by  belt  or  gearing  or  having 
its  armature  mounted  on  the  alternator  shaft.     (See  Fig.  1137.) 


FIG.  1458.— ALTERNATOR    WITH    DIRECT-CONNECTED    EXCITER. 

1458.     Why  is  the  exciter  armature  sometimes  mounted  on  the 
same  shaft  with  the  alternator ? 

The  early  practice  was  to  drive  alternators  by  belts  and  to  belt  the 


ALTERNATING  CURRENTS. 


361 


exciter  from  the  alternator,  the  exciter  making  more  revolutions  per 
second,  the  high  speed  keeping  down  its  cost.  Trouble  was  found  in 
keeping  the  proper  tension  on  the  exciter  belt,  especially  when  ad- 
justing the  main  belt.  Most  of  the  difficulties  were  eliminated  when 
the  exciter  was  mounted  on  the  alternator,  especially  when  directly 
driven.  When  the  alternator  is  large  enough  to  be  directly  connected 
to  the  source  of  power  the  exciter  is  generally  driven  by  its  own  power. 

1459.     Why  are  alternators  generally  made  with  revolving  fields? 

The  armature  windings  can  be  better  insulated,  having  more  space 
and  not  being  subject  to  so  severe  mechanical  strains  This  allows 
alternators  to  be  wound  for  higher  voltages,  thus  obviating  the  use 
of  step-up  transformers  in  some  cases.  The  only  moving  contacts 


FIG.  1459.— STATORS    OF    REVOLVING    FIELD   ALTERNATORS,    SHOWING 
GROUP    AND    UNIFORM    WINDINGS. 

and  other  exposed  conductors  are  those  supplying  the  field  winding, 
which  is  for  low  potential,  125  to  500  volts.  The  revolving  field  has 
greater  flywheel  effect,  which  is  sometimes  desirable. 


362 


ELECTRICAL   CATECHISM. 


1460.  What  sort  of  machines  have  both  Held  and  armature 
stationary? 

The  so-called  inductor  machines  have  no  moving  wire  whatever. 
Examples  are  the  Mordey  (English),  Stanley  and  Warren  alterna- 


FIG.  1460A.— INDUCTOR  ALTERNATOR  (ARMATURE  OPENED). 


FIG.  1460B.— INDUCTOR  ALTERNATOR  (SECTIONS). 

tors.  The  figures  show  the  Stanley  ("  S.K.C."  or  Stanley-Kelly- 
Chesney ")  inductor  alternator  as  made  by  the  General  Electric 
Company.  The  stationary  field  coil  o  sends  magnetic  flux  through 


ALTERNATING  CURRENTS.  363 

the  path  indicated  by  dash-dotted  line,  passing  through  the  steel 
magnet  bars  F  and  the  two  slotted  laminated  armature  cores 
of  the  stator,  and  through  the  laminated  steel  poles  N  and  cast-steel 
spider  5*  of  the  revolving  element  or  "  inductor."  The  armature 
windings  consist  of  two  sets  or  crowns  of  coils,  each  crown  including 
two  (or  three  for  three-phase)  interspaced  series  of  coils,  one  coil 
per  phase  per  pole. 


FIG.  1460c.— INDUCTOR    ALTERNATOR    FIELD    COIL    AND    ROTOR. 

1461.  How  is  the  E.M.F.  induced  in  inductor  machines  f 

The  magnetic  field  always  passes  through  a  given  armature  coil 
in  the  same  direction,  but  the  strength  of  the  field  through  the  coil 
varies  according  to  the  position  of  the  moving  projecting  pole.  This 
causes  a  regular  periodic  rise  and  fall  in  the  magnetic  flux  through 
each  coil.  The  two  sets  of  coils  in  each  crown  are  interspersed  so 
that  the  magnetic  field  through  one  set  is  strongest  when  the  field 
through  the  other  set  is  weakest.  The  E.M.F's  in  the  two  sets  are 
therefore  90  degrees  apart,  or  "  in  quadrature."  In  the  three-phase 
machines,  the  space  is  divided  so  as  to  hold  three  equally  spaced  coils 
per  pole. 

1462.  How  may  the  voltage  of  an  alternator  be  regulated? 

By  varying  the  rate  of  change  of  the  number  of  lines  of  force  pass- 


364 


ELECTRICAL  CATECHISM. 


ing  through  the  armature  coils.  The  general  principles  are  similar  to 
those  for  the  regulation  of  direct-current  machines,  for  which  see 
Nos.  1158  to  1170.  The  methods  applicable  to  alternators  are  varia- 
tion of  the  number  of  active  armature  wires  and  of  the  field  strength, 
speed  variation  being  barred  by  motors  requiring  constant  frequency. 

1463.  Ho^v  can  the  number  of  active  wires  be  changed  in  the 
armature  of  an  alternator? 

On  revolving  field  alternators,  it  is  practicable  to  arrange  a  switch 
or  "  regulator  head  "  with  "  taps  "  brought  out  from  some  of  the 


0  DO  DO  00  0 


FIG.   146a— METHODS   OF  GETTING   SEVERAL  VOLTAGES    FROM   ONE 
ALTERNATOR. 

end  coils,  or  from  different  points  in  the  winding  of  a  single-coil 
transformer  shunted  around  one  or  more  armature  coils. 

1464.  In  what  ivays  may  the  field  strength  of  an  alternator  be 
changed? 

By  varying  the  current  through  the  field  coils.  The  other  methods 
sometimes  used  with  continuous-current  machines,  namely,  of  vary- 
ing the  number  of  turns  in  the  field  coils  or  of  changing  the  re- 
luctance of  the  magnetic  circuit,  are  not  generally  applicable  because 
of  the  large  number  of  poles  in  the  alternator  field. 


ALTERNATING  CURRENTS. 


365 


1465.  In  what  ways  may  the  field  current  of  an  alternator  be 
changed? 

By  varying  the  voltage  of  the  exciter  dynamo,  by  varying  the  re- 
sistance in  the  alternator  field  circuit  and  also,  if  the  machine  is  of 
the  composite  type,  by  varying  the  current  through  the  rectifier  and 
series  coils. 

1466.  How  is  the  voltage  of  the  exciter  varied? 

It  is  common  to  use  a  shunt  dynamo  for  the  exciter  and  to  control 
its  voltage  by  a  rheostat  in  the  field  circuit.  A  second  rheostat  in  the 


FIG.  1466.— ALTERNATOR  WITH  EXCITER  AND  RHEOSTATS. 

field  circuit  of  the  alternator  allows  closer  regulation,  and  also  al- 
lows independent  regulation  of  two  or  more  alternators  supplied 
from  the  same  exciter.  Large  stations  frequently  have  several  ex- 
citers feeding  into  special  busbars,  from  which  the  alternators  are 
supplied,  each  having  its  own  field  rheostat  and  field  switch. 

1467.     How  was  the  Heisler  alternator  regulated? 

In  the  Heisler  machines  formerly  used  for  series  incandescent 
lighting,  the  exciter  was  a  series  dynamo,  the  voltage  of  the  exciter 
and,  consequently,  the  current  through  the  alternator  field  being 
regulated  entirely  by  shifting  the  brushes,  a  method  common  with 
arc-light  dynamos.  A  similar  method  was  used  with  some  European 
alternators.  Since  the  Heisler  machine  was  a  two-phaser,  supplying 
two  series  circuits  in  which  it  was  necessary  to  maintain  the  current 
of  constant  average  value,  an  auxiliary  regulator  was  necessary  to 
govern  each  current  independently.  This  consisted  of  an  adjustable 
rheostat  in  each  circuit,  with  an  ingenious  automatic  device  that  cut 
resistance  in  or  out  as  might  be  necessary.  There  was  a  further 
auxiliary  attachment  that  kept  the  exciter  brushes  in  such  a  position 
that  the  E.M.F.  of  the  alternator  was  just  sufficient  to  keep  the  cur- 
rent of  the  proper  strength  in  the  longest  circuit.  This  machine  had 
the  reputation  of  being  about  the  only  alternator  of  the  older  genera- 
tion that  paid  dividends  to  the  station  manager,  a  result  due  to  the 
absence  of  the  imperfect  transformers  of  early  days. 


366 


ELECTRICAL   CATECHISM. 


1468.     W 'hat  is  an  alternator  of  the  composite  type? 

A  composite  alternator  is  similar  to  a  compound-wound,  continu- 
ous dynamo  in  that  it  has  two  field  windings.  In  addition  to  the 
regular  field  coils  which  carry  the  main  magnetizing  current  from  the 


FIG.  1468.— COMPOSITE  ALTERNATOR. 

exciter,  there  is  a  second  winding  upon  two  or  upon  all  of  the  pole 
pieces,  carrying  a  rectified  current  from  the  alternator  which 
strengthens  the  field  to  balance  the  losses  in  the  machine,  and  also, 
if  so  desired,  the  losses  on  the  line. 

1469.     How  is  the  alternating  current  rectified? 

The  alternator  shaft  carries  a  commutator  having  as  many  seg- 
ments as  there  are  poles  in  the  field.  Alternate  bars  are  coupled  and 
brushes  are  adjusted  so  that  both  change  simultaneously  from  one 
set  of  bars  to  the  other.  When  the  commutator  is  connected  to  the 
armature  circuit  the  brushes  may  be  adjusted  so  as  to  change  from 
one  set  of  bars  to  the  other  at  the  same  instant  when  the  armature 
current  changes  direction.  The  brushes  will  then  take  off  a  current 
that  pulses,  but  always  flows  in  the  same  direction. 


ALTERNATING  CURRENTS. 


367 


FIG.  il®.— CIRCUITS  OF  COMPOSITE  ALTERNATOR 

1470.     Hoiv  is  the  rectifier  connected  into  the  circuit? 

Sometimes  it  is  connected  directly  in  series  with  the  armature, 
one  set  of  commutator  segments  being  connected  directly  to  one  end 
of  the  armature  winding,  while  the  other  set  of  bars  is  connected 


AUXILIARY  FIELC 


American  Electrician 


FIG.    1470.— CIRCUITS   OF   POLYPHASE   COMPOSITE   ALTERNATORS. 

to  the  outside  circuit  through  a  collecting  ring  and  brush.  Some- 
times an  adjustable  shunt  is  connected  around  the  commutator,  as 
suggested  in  Fig.  1469,  so  that  only  part  of  the  current  is  rectified. 
In  other  cases,  the  whole  current  is  rectified,  but  only  part 
goes  through  the  field  coils  while  the  remainder  goes  through 
an  adjustable  shunt.  Sometimes  the  current  for  the  com- 


368 


ELECTRICAL   CATECHISM. 


mutator  is  taken  from  the  secondary  of  a  transformer  attached  to  the 
armature  spider  and  having  its  primary  connected  in  series  with  the 
armature  windings;  this  scheme  is  to  avoid  having  the  field  coils 
connected  with  the  high-voltage  circuit.  The  figures  show  the  cir- 
cuits for  two  and  three-phase  composite  alternators.  In  the  three- 
phase  machine,  one  of  the  coils  is  reversed  so  that  the  instantaneous 
sum  of  the  three  currents  is  no  longer  zero,  since  the  currents  are  60 
degrees  apart ;  the  combination  of  two  currents,  OA  and  OB,  gives 
a  resultant,  OD,  which,  combining  with  the  third  current,  OC,  gives 
the  final  resultant,  OE,  which  is  the  effective  current  in  the  trans- 
former, inducing  the  current  which  goes  to  the  commutator  and 
thence  to  the  auxiliary  field  coils. 

1471.  Can  several  circuits  be  supplied  at  different  voltages  by  the 
same  alternator? 

This  may  be  done  by  the  use  of  "boosters"  or  adjustable  trans- 
formers connected  in  the  various  lines.  Alternators  having  station- 
ary armature  coils  may  be  provided  with  a  number  of  terminals  so 
that  any  circuit  may  be  connected  around  any  desired  number  of 
armature  coils. 

1472.  What  is  a  booster ? 

When  speaking  of  continuous  currents,  a  booster  is  a  small  dy- 
namo connected  in  series  with  a  line  so  that  its  E.M.F.  is  added  to 


FIG.  1472A.-DIAGRAMS  OF  BOOSTER  TRANSFORMER. 

the  voltage  across  the  terminals.    When  speaking  of  alternating  cur- 
rents, the  booster  is  a  special  transformer  whose  secondary  is  con- 


ALTERNATING  CURRENTS. 


369 


nected  in  the  line  tovraise  or  lower  the  voltage.  The  secondary  coil 
has  several  terminals  for  adjustment  and  the  primary  coil  is  con- 
nected through  a  reversing  switch  so  that  the  secondary  will  either 
raise  or  lower  the  voltage  on  the  line.  Diagrams  of  the  Westing- 
house  booster  are  given  in  the  figures.  The  General  Electric  Com- 


FIG.  1472s.— SERIES   INCANDESCENT   REGULATOR. 

pany  has  two  kinds  of  booster  or  compensator.  That  intended  for 
handling  currents  of  considerable  magnitude  has  a  primary  and  a 
secondary  coil  at  right  angles  to  each  other  and  enclosing  a  common 
laminated  iron  core  with  an  adjustable  center.  By  shifting  the  posi- 
tion of  the  central  core,  the  inductive  relation  of  the  two  coils  is  modi- 
fied so  that  the  voltage  induced  in  the  secondary  is  varied  in  amount 
and  direction.  For  smaller  currents,  such  as  3.5  or  5.5  amperes, 
used  for  operating  incandescent  street  lamps  in  series,  they  used  a 
compensator  similar  to  that  shown  in  the  first  figure,  except  that  in- 
stead of  reversing  the  primary  to  raise  or  lower  the  voltage,  the  sec- 
ondary terminals  are  arranged  with  two  sliding  contacts,  H  and  G, 
which  can  move  relatively  past  one  another  so  as  to  raise  or  lower 
the  voltage  any  desired  amount;  when  H  and  G  are  on  the  same 


370 


ELECTRICAL   CATECHISM. 


block,  the  voltage  on  the  line  is  the  same  as  that  on  the  switchboard. 
Their  more  recent  practice  is  to  use  constant-current  transformers 
similar  to  those  used  for  series  arc  lighting  (see  No.  1426). 

1473.  What  is  a  choke  coil? 

A  choke  coil  consists  of  a  coil  of  wire  surrounding  a  laminated  iron 
core.  It  usually  has  a  number  of  terminals  so  that  the  number  of 
turns  of  wire  in  circuit  may  be  adjusted,  or  in  some  cases  part  or  all 
of  the  iron  core  is  movable  so  as  to  change  the  inductance  of  the  cir- 
cuit. The  coil  is  connected  in  series  with  any  circuit  desired. 

1474.  For  what  purposes  is  a  choke  coil  used? 

For  reducing  the  voltage  and  current  in  a  circuit.  The  self-induc- 
tion of  the  choke  coil  acts  as  a  C.E.M.F.  or  "back  voltage"  opposing 
the  impressed  or  line  voltage,  and  thus  reducing  the  current.  For 
example,  a  choke  coil  may  be  connected  in  series  with  a  group  of  in- 
candescent lamps  in  order  to  dim  them.  Choke  coils  are  being  used 
to  a  considerable  extent  for  maintaining  constant  current  in  a  series 
of  arc  lamps  operated  from  a  constant-potential  alternator,  the  in- 
ductance of  the  coil  being  adjusted  to  compensate  any  changes  of  re- 
sistance due  to  the  feeding  of  the  arc-lamp  carbons. 

1475.  What  advantage  has  a  choke  coil  over  a  resistance  coil? 
The  resistance  coil  reduces  the  current  on  a  line  by  increasing  the 

resistance,  but  at  the  same  time  absorbing  a  considerable  part  of  the 
energy.  For  example,  suppose  a  group  of  lamps  on  a  i  lo-volt  circuit 
takes  10  amps.,  or  noo  watts.  The  resistance  of  the  lamps  is 

R  =  E/I  =  1 10/10  =  1 1  ohms. 

Now,  suppose  a  resistance  of  n  ohms  is  inserted  in  series  with  the 
lamps,  and  assume  for  simplicity  that  the  considerable  change  in 
the  resistance  of  the  lamps  (see  Nos.  345  and  814)  is  negligible. 


K> 

0 

6 

ce 

8 

110 

MM 

«/> 

H 
DC 
u 

VOtT5 

^ 

£ 

—  —         ^ 

s 

•; 

9"' 

110  VOLTS 

FIG.  1475.— EFFECT  OF  RESISTANCE. 


The  total  resistance  is  R  —  1  1 
I  =  E/R=  110/22  =  5  amperes. 


1  1  =  22    ohms.     The  current  is 
The  total  energy  is  the  sum  of 


ALTERNATING  CURRENTS.  371 

that  in  lamps  and  in  resistance,  or,  energy  is  FR  =  5  X  5  X  22  =  550 
watts.  The  energy  used  in  the  lamps  is  W  =  IR  =  5X5X  1 1  = 
275  watts,  or  only  half  the  total  energy,  while  the  balance  of  275 
watts  is  absorbed  by  the  resistance.  By  inserting  an  equal  resistance, 
the  energy  used  in  the  lamps  is  reduced  to  one-fourth  while  the  total 
energy  taken  by  the  circuit  is  reduced  to  one-half.  In  other  words, 
while  the  added  resistance  reduces  the  total  energy  taken  by  the  cir- 
cuit, it  also  wastes  a  large  part  of  what  is  taken.  On  the  other  hand, 
the  choke  coil  also  reduces  the  total  energy  taken  by  the  circuit,  but 
does  not  waste  much  of  it. 

1476.  Is  a  choke  coil  suitable  for  direct  currents? 

No.  It  is  used  only  with  alternating  currents.  With  direct  cur- 
rent it  would  act  only  as  a  very  low  resistance  and  would  not  ma- 
terially reduce  the  voltage  or  the  current. 

1477.  How  can  a  choke  coil  reduce  the  current  without  wasting 
energy? 

The  choke  coil  sets  up  a  C.E.M.F.  which  not  onty  reduces  the  cur- 
rent, but  also  causes  the  current  waves  to  lag  behind  the  E.M.F. 
waves.  The  energy  actually  used  is  the  product  of  the  current  by 
the  component  of  the  E.M.F.,  which  is  in  the  same  phase.  This  may 
be  illustrated  by  an  enclosed  alternating  current  arc  lamp  taking  5 
amperes  from  a  H2-volt  circuit.  The  arc  itself  takes  about  80  volts, 
acting  partly  as  a  C.E.M.F.  in  phase  with  the  current  and  partly  as 
an  ohmic  resistance.  The  coils  in  series,  including  the  choke  coil 
and  the  magnet  coils  which  control  the  feeding  of  the  arc  electrodes, 
take  about  52  volts  nearly  in  quadrature  with  the  current  and  with 
the  voltage  at  the  arc.  The  apparent  energy  taken  is  80.  X  5  =  400 
volt-amperes  by  the  arc,  52  X  5  =  260  volt-amperes  by  the  coils  and 
112  X  5  —  560  watts  by  the  whole  lamp.  The  actual  energy  is  about 
395  watts  by  the  arc  and  about  434  watts  by  the  whole  lamp,  the  dif- 
ference being  39  watts  absorbed  by  the  coils.  If  the  difference  of  32 
volts  between  the  80  at  the  arc  and  the  112  at  the  terminals  had  been 
absorbed  by  resistance,  the  energy  loss  would  have  been  32  X  5  — 160 
watts  instead  of  the  39  watts  actually  absorbed. 

1478.  Is  not  the  energy  of  an  alternating  current  equal  to  the 
product  of  amperes  by  volts? 

Sometimes  it  is,  but  usually  is  not.  When  there  is  no  lag  between 
the  current  and  voltage,  that  is,  when  the  waves  of  current  and  pres- 
sure reach  corresponding  values  simultaneously,  both  being  at  zero 


372 


ELECTRICAL   CATECHISM. 


at  the  same  time  and  both  being  at  positive  maximum  or  other  cor- 
responding values  at  the  same  time,  the  simple  product  of  amperes 
by  volts  gives  watts.  This  is  illustrated  in  the  figure,  which  repre- 
sents a  circuit  without  induction,  such  as  a  group  of  incandescent 


FIG.  1478.— ENERGY  IN   NON-INDUCTIVE   CIRCUIT. 

lamps,  in  which  the  current  is  in  phase  with  the  voltage.  The  energy 
at  any  instant  is  the  product  of  current  by  voltage  at  that  instant. 
The  upper  curve  is  plotted  from  the  products  of  corresponding  values 


FIG.  1479.— POSITIVE  AND  NEGATIVE  ENERGY  IN  INDUCTIVE 
CIRCUIT. 

of  current  and  voltage.  The  area  enclosed  between  the  watt  curve 
and  the  base  line  represents  the  energy  in  the  circuit.  In  the  second 
part  of  the  cycle,  the  current  and  voltage  are  both  negative,  but  the 
energy  is  positive,  since  the  product  of  two  negatives  is  always  posi- 
tive. Distortion  of  either  wave  acts  somewhat  like  phase  difference. 

1479.  How  can  the  energy  of  an  alternating  current  be  different 
from  the  simple  product  of  amperes  by  volts? 

If  the  circuit  has  self-induction  or  capacity,  as  is  generally  the 
case,  the  current  waves  are  behind  or  ahead  of  the  voltage  waves,  and 


ALTERNATING  CURRENTS.  373 

the  true  energy  is  less  than  the  simple  product  of  volts  by  amperes. 
This  is  illustrated  in  Figs.  1479  and  1481,  which  have  current  and 
voltage  curves  equal  to  those  of  Fig.  1478,  but  which  have  different 
watt  curves.  If  the  curves  of  current  and  voltage  are  not  in  the  same 
phase,  one  will  sometimes  have  a  positive  value  when  the  other  is 
negative,  the  result  being  that  their  product  is  negative  for  part  of 
each  cycle,  as  shown  in  Figs.  1479  and  1481.  The  area  of  the  loop 
on  the  lower  side  between  the  base  line  and  the  watt  curve  represents 
negative  energy ;  that  is,  energy  that  is  returned  to  the  line  or  source 
of  supply. 

1480.  How  can  a  line  deliver  current  or  energy  back  to  a  gen- 
erator? 

During  parts  of  the  cycle,  the  pressure  on  the  line  may  be  higher 
than  that  from  the  generator,  and  so  send  current  back.  A  some- 
what similar  case  is  that  of  a  steam  engine ;  during  part  of  the  stroke, 
the  piston  drives  the  flywheel ;  then  the  wheel  carries  the  system  over 
the  dead  center. 

1481.  How  can  an  alternating  current  be  wattless  or  without 
energy? 

This  would  occur  in  the  extreme  case  when  the  current  and  E.M.F. 
curves  are  90  degrees  apart,  one  being  at  zero  when  the  other  is  at  a 
maximum,  as  indicated  in  the  figure.  The  negative  loop  in  the  watt 


FIG.  1481.-WATTLESS  CURRENT. 

curve  exactly  equals  the  positive  loop,  and  the  line  returns  as  much 
energy  as  it  receives.  This  extreme  condition  never  happens,  but  is 
closely  approximated  when  a  circuit  has  great  self-induction  and 
small  resistance,  as  in  the  case  of  the  primary  coil  of  a  transformer 
when  the  secondary  circuit  is  open. 

1482.  Is  it  correct  to  say  that  part  of  an  alternating  current  is 
wattless  and  that  the  rest  has  energy  or  power? 

Yes.  The  whole  current  may  be  considered  as  being  made  up  of 
two  parts,  one  (called  the  "energy  component,"  or  "useful  current") 
which  is  in  phase  with  the  impressed  or  line  E.M.F.,  and  one  (called 


374  ELECTRICAL  CATECHISM.. 

the  "wattless  component,"  "idle  current"  or  "wattless  current") 
which  is  90  degrees  behind  or  ahead  of  it.  (When  the  capacity  in  the 
circuit  is  more  effective  than  the  induction,  the  current  leads  the 
line  pressure  and  the  wattless  component  of  the  current  is  90  degrees 
ahead  of  the  active  component.  This  condition  is  reversed  when,  as 
usual,  the  induction  is  stronger  than  the  capacity).  Although  there 
is  only  one  current  in  a  series  circuit,  it  may  be  thought  of  as  being 
resolved  into  components,  whose  sum  properly  taken  equals  the 
whole.  This  is  strictly  analogous  to  the  composition  and  resolution 
of  forces.  The  ratio  of  the  active  component  to  the  total  current  is 
called  the  "cosine  of  the  angle  of  lag,  or  of  lead." 

1483.  What  is  meant  by  the  "power  factor"? 

The  power  factor  is  the  ratio  of  the  true  watts  to  the  apparent 
watts  or  volt-amperes.  It  may  be  considered  as  the  ratio  of  the 
"useful"  to  the  total  current  in  the  circuit.  As  explained  in  No 
1482,  the  current  in  a  circuit  may  be  considered  as  made  up  of  two 
components,  of  which  only  one  is  useful.  The  true  energy  in  the  cir- 
cuit is  the  product  of  the  line  pressure  by  the  active  component  of  the 
current,  which  equals  the  product  of  volts  by  amperes  by  the  cosine 
of  the  angle  of  lag.  The  power  factor  is  never  greater  than  unity 
and  is  generally  less.  With  old  style  open  arc  lamps  the  power  factor 
was  sometimes  as  low  as  0.30.  With  the  modern  enclosed  arc  lamps, 
the  power  factor  varies  from  0.45  to  0.85.  With  induction  motors,  it 
varies fromo. 30 too.95.  Synchronous  motors  reach  i.oo  power  factor. 

1484.  Does  the  line  voltage  have  a  wattless  component ? 

The  power  equals  the  product  of  current  by  voltage  by  cosine  of 
angle  of  lag.  This  may  be  considered  as  made  up  of  the  product  of 
current  by  cosine  times  voltage,  or  of  the  product  of  voltage  by  cosine 
times  current.  In  other  words,  either  the  current  or  voltage  may  be 
considered  as  resolved  into  components. 

1485.  Why  do  writers  on  alternating  currents  use  so  many  dia- 
grams with  circles  or  triangles? 

These  are  generally  used  to  explain  or  to  determine  the  relation 
between  the  various  currents  and  E.M.F's  that  must  be  considered 
in  discussing  A.  C.  problems.  With  continuous  currents  the  rela- 
tions between  current  and  E.M.F.  are  very  simple,  but  in  dealing 
with  alternating  currents  and  E.M.F's,  one  may  be  at  its  greatest 
value  at  a  time  when  another  is  at  its  zero  or  any  intermediate  value. 
When,  for  example,  the  current  and  E.M.F.  do  not  reach  their  high- 
est values  at  the  same  time,  as  in  Fig.  1499  or  15023  (which  represent 
cases  where  self-induction  causes  the  current  to  lag  behind  the 


ALTERNATING  CURRENTS  375 

pressure),  the  watts  can  not  be  determined  by  simpiy  multiplying 
current  by  volts,  since  that  would  give  too  large  a  result.  By  the  use 
of  geometry,  we  can  easily  determine  the  part  of  the  E.M.F.  which  is 
in  phase  with  the  current,  and  can  then  get  the  watts  by  multiply- 
ing the  current  by  that  part  of  the  E.M.F.  To  do  this,  we  must 
know  how  to  resolve  the  whole  E.  M.F.  into  its  elements. 
By  a  simple  geometric  method,  we  can  find  what  will  be  the 
result  of  adding  two  or  more  E.M.F's  or  currents  that  have  any  dif- 
ference of  phase.  By  reversing  the  same  process,  we  can  take  any 
E.M.F.  or  current  and  find  what  two  or  more  elements  might  have 
been  combined  to  make  it,  or  if  we  know  one  of  the  elements,  we  can 
easily  find  the  other.  This  process  is  an  application  of  what  is  gen- 
erally called  "the  resolution  of  forces." 

1486.  How  can  the  resultant  of  two  or  more  forces  be  determined 
graphically ? 

The  forces  may  be  represented  in  direction  and  amount  by  straight 
lines  drawn  in  suitable  directions  and  of  such  length  as  to  represent 
to  any  convenient  scale  the  amount.  For  example,  let  A  and  B  in  the 
figures  represent  two  horses  pulling  in  the  directions  shown,  the 
length  of  the  lines  representing  the  strength  with  which  each  horse 
pulls.  Then  their  combined  pull  equals  that  which  might  be  given 


FIG.  I486.— COMPOSITION  OF  FORCES. 


by  a  third  horse  pulling  in  the  direction,  C,  and  to  the  amount  indi- 
cated by  length  of  line,  C.  The  direction,  C,  is  found  by  drawing 
lines  through  A  parallel  to  B  and  through  B  parallel  to  A.  The  line 
drawn  from  the  intersection  of  A  and  B  to  the  intersection  of  the 
lines  parallel  to  them  represents  in  direction  and  in  amount  the  re- 
sultant When  there  are  more  than  two  component  forces,  these 
may  be  combined  in  pairs  and  the  resultant  of  the  pairs  may  then  be 
treated  as  components.  It  is  clear  from  the  last  figure  at  the  right 
that  if  the  two  component  forces  are  in  exactly  the  same  direction, 
their  resultant  is  in  the  same  direction  and  equals  their  arithmetical 
or  algebraic  sum.  But  if  the  component  forces  are  not  in  the  same 
direction,  their  geometrical  or  trigonometrical  sum  must  be  taken. 
For  example,  in  the  figure  at  the  left,  if  A  and  B  were  in  the  same 


376  ELECTRICAL   CATECHISM. 

direction,  their  sum  would  be  o  c\  as  shown  in  the  lower  part  of  the 
figure,  instead  of  the  smaller  resultant,  o  c. 

1487.  How  can  a  force  be  resolved  into  its  components? 

The  simplest  way  is  by  geometry  or  trigonometry.  Any  force  or 
resultant  may  be  resolved  into  any  desired  number  of  components, 
and  any,  or  all,  of  the  components  may  be  equal  to,  or  greater  or 
smaller  than  the  resultant,  as  seen  in  Fig.  1486.  There  may  be  any 
number  of  pairs  of  components  derived  from  any  one  resultant,  and 
it  is  necessary  to  fix  certain  conditions  if  one  is  to  obtain  a  definite 
desired  pair.  Since  each  resultant  is  determined  by  its  length  and 
direction,  each  solution  involves  two  directions  and  two  lengths.  If 
both  lengths  are  given,  or  both  directions,  or  one  direction  and  one 
length,  the  solution  is  easy. 

1488.  How  can  the  components  be  determined  when  their  di- 
rections are  known? 

Draw  the  resultant  first,  as  O  C,  of  Fig.  1486.  Then  from  O  draw 
lines  in  the  proper  directions,  but  of  indefinite  length.  Then  from  C 
draw  lines  parallel  to  O  A  and  to  O  B.  The  points  where  these 
cross  determine  the  lengths  of  O  A  and  O  B.  This  problem  has  only 
the  one  solution. 

1489.  How  can  the  components  be  determined  ^vhen  their  lengths, 
but  not  their  directions,  are  known? 

Draw  the  resultant,  O  C,  first,  as  in  Fig.  1489.  From  each  end 
as  a  center  draw  two  circles,  each  with  a  radius  equal  to  one  of  the 
two  forces.  From  O  and  C  draw  lines  to  the  points  where  the  larger 
circle  drawn  from  O  is  cut  by  the  smaller  circle  drawn  from  C.  Draw 
lines  parallel  to  these  from  O  and  C  to  the  point  where  the  smaller 
circle  drawn  from  O  is  cut  by  the  larger  circle  drawn  from  C.  Then 


FIG.   1489.-DETERMINATION    OF    COMPONENTS. 

O  A  and  O  B  are  the  components  sought.  It  is  seen  also  that  another 
solution  may  be  had  by  drawing  a  parallelogram  on  O  B'  C  A',  so  that 
this  problem  has  two  solutions,  unless  the  two  components  are  equal. 


ALTERNATING  CURRENTS. 


377 


1490.  How  can  the  components  be  determined  when  the  direction 
of  one  and  the  length  of  the  other  are  known? 

Draw  the  resultant,  0  C,  as  in  Fig.  1490.  From  0  draw  a  line  of 
indefinite  length  in  the  known  direction  'of  one  force.  From  C  draw 
a  circle  with  a  radius  representing  on  the  proper  scale  the  known 
force.  Draw  a  radius  from  C  to  the  point  where  the  circle  crosses 


FIG.  1490.— DETERMINATION  OF  COMPONENTS. 


the  line.  From  C  also  draw  a  line  parallel  to  O  A,  and  from  O  draw 
a  line  parallel  to  C  A.  Then  O  A  and  O  B  are  the  two  components 
desired.  It  is  seen  that  the  line  may  cut  the  circle  in  two  places,  giv- 
ing the  second  parallelogram,  O  A'  C  Br,  and  the  two  components, 
O  A'  and  OB'.  If  the  line  does  not  cut  or  touch  the  circle  at  all, 
there  is  no  solution. 

1491.  Of  what  value  is  this  geometry  for  studying  alternating 
currents? 

It  enables  one  to  solve  many  problems  that  otherwise  would  be 
very  difficult.  For  example,  if  one  knows  the  amperes,  volts  and 
angle  of  lag,  it  is  easy  to  determine  the  watts  graphically  or  by  a 
geometric  method  similar  to  the  above.  Or,  knowing  the  amperes 
and  angle  of  lag  between  the  voltage  and  current,  it  is  easy  to  deter- 
mine the  amount  of  the  wattless  component  of  the  current. 

1492.  How  can  one  determine  the  wattless  component  of  the  cur- 
rent when  the  angle  of  lag  is  known? 

This  may  be  determined  by  the  method  given  in  No.  1487.    The 


0  A 

FIG.  1492.— DETERMINATION  OF  WATTLESS  COMPONENT. 

current  is  to  be  resolved  into  two  components,  one  representing  the 
active  current  in  phase  with  the  E.M.F.,  and  the  other  representing 


378  ELECTRICAL  CATECHISM. 

the  wattless  current  at  right  angles,  or  90  degrees  behind  it.  From 
any  point,  0,  taken  as  the  origin,  draw  (as  in  Fig.  1492)  an  indefinite 
horizontal  line,  O  X,  representing  the  E.M.F.,  and  the  direction  of 
the  active  component  of  the  current.  From  O  draw  another  line, 
making  an  angle,  COX,  equal  to  the  angle  by  which  the 
current  lags  behind  the  E.M.F.  Lay  off  a  distance,  O  C,  on  this  sec- 
ond line  which  shall  represent  on  a  suitable  scale  the  total  current. 
Now  it  is  desired  to  resolve  O  C  into  two  components,  one  of  which 
shall  be  in  the  direction,  0  X,  and  the  other  at  right  angles  to  it. 
From  C  draw  a  line,  C  B,  parallel  to  O  X,  and  another  line  perpen- 
dicular to  it.  Draw  a  perpendicular  line,  O  Y,  from  O.  Then  we 
have  the  parallelogram  of  forces,  O  A  C  B,  and  the  components  of 
O  C  are  O  A  and  O  B.  In  other  words,  O  C  represents  the  actual 
current  flowing,  O  A  is  the  component  of  it  which  is  active  or  in 
phase  with  the  E.M.F.,  and  O  B  Is  the  wattless  component. 

1493.  What  is  meant  by  the  "angle  of  lag?" 

This  is  a  convenient  method  of  indicating  the  "difference  of  phase" 
between  the  current  and  E.M.F.  An  alternating  current  is  usually 
more  or  less  out  of  phase  with  the  E.M.F.  causing  it ;  that  is,  the  cur- 
rent reaches  its  greatest  or  maximum  value  after  the  E.M.F.  has 
reached  its  maximum.  Current  and  E.M.F.  each  take  the  same 
amount  of  time  to  complete  a  cycle  or  complete  alternation,  but  the 
current  is  usually  a  little  later  than  corresponding  values  of  the 
E.M.F.  causing  it.  In  order  to  measure  the  amount  of  this  dragging 
behind,  the  time  of  a  complete  cycle  is  considered  as  being  divided 
into  360  equal  parts  or  degrees,  the  most  convenient  place  to  begin 
numbering  being  the  point  where  the  E.M.F.  or  current  passes 
through  zero.  The  angle  of  lag  between  current  and  E.M.F.  is 
shown  by  the  distance  (measured  in  degrees)  between  corresponding 
values ;  for  instance,  the  distance  between  the  points  where  the  two 
curves  cross  the  base  line  in  the  same  direction,  as  at  A  and  B,  in  the 
upper  diagram  of  Fig.  1499. 

1494.  What  causes  the  lag  of  current  behind  the  E.M.F.  f 

The  self-inductance  of  the  circuit  causes  the  current  to  lag  behind 
the  E.M.F.  This  may  be  counteracted  by  the  "capacity"  of  the  cir- 
cuit, which  has  a  tendency  to  make  the  current  "lead"  the  current. 

1495.  How  does  self-inductance  cause  the  current  to  lag  behind 
the  E.M.F.?. 

In  any  circuit  having  self -inductance  there  is  a  tendency  to  prevent 
changes  in  the  strength  of  the  current.  Every  current  is  surrounded 
by  a  magnetic  field  which  is  caused  by,  and  is  proportional  to,  the 


ALTERNATING  CURRENTS. 


379 


strength  of  the  current.  When  two  parts  of  the  circuit  are  near  to- 
gether, so  that  one  is  in  the  magnetic  field  of  the  other,  any  change  in 
the  strength  of  the  current  causes  a  corresponding  change  in  the  mag- 
netic field  and  so  sets  up  an  E.M.F.  in  the  other  wire.  Each  part 
reacts  on  the  other,  and  in  a  coil  the  induced  E.M.F.  is  proportional 
to  the  square  of  the  number  of  turns  of  wire.  This  induced  E.M.F. 
is  in  a  direction  to  oppose  the  change  in  the  current  and  is  therefore 
called  a  "counter  E.M.F."  In  the  case  of  an  alternating  current,  this 
therefore  causes  the  current  to  reach  its  maximum  value  a  little  later 
than  the  line  or  impressed  E.M.F. ;  the  same  tendency  also  causes  the 
current  to  hold  up  after  the  E.M.F.  has  diminished.  Thus  the  re- 
sult of  the  E.M.F.  of  self-inductance  is  to  make  the  current  lag  be- 
hind the  principal  or  impressed  E.M.F. 

1496.  Is  the  E.M.F.  of  self-inductance  directly  opposed  to  the 
impressed  E.M.F.? 

No.  It  is  about  90  degrees  behind  the  current,  and  therefore  is 
nearly  opposite  the  impressed  or  line  E.M.F.  The  E.M.F.  of  self- 
inductance  is  at  every  instant  proportional  to  the  rate  of  change  of 
the  current.  When  the  current  is  at  its  maximum  value,  it  is  chang- 


FIG.  1496.-RELATION  OF  CURRENT  AND    C.E.M.F. 

ing  least  rapidly — in  fact,  has  a  constant  value  for  an  instant — and 
therefore  the  induced  E.M.F.  at  that  instant  is  zero;  when  the  cur- 
rent is  zero  it  is  changing  most  rapidly,  and  therefore  the  induced 
E.M.F.  is  at  its  maximum  value.  The  E.M.F.  of  self-inductance  is 
therefore  90  degrees  behind  the  current,  as  is  indicated  in  the  figure. 

1497.  What  governs  the  angle  of  lag  between  the  impressed  or 
line  E.M.F.  and  the  current? 

The  lag  is  governed  by  the  relative  values  of  the  various  E.M.F's 
in  the  circuit.  In  a  circuit  having  resistance  and  self-inductance,  but 
no  capacity  worthy  of  notice,  three  E.M.F's  must  be  considered : 
that  on  the  line,  or  the  impressed ;  that  due  to  self-inductance ;  that 
due  to  the  "  ohmic  drop,"  which  equals  current  times  effective  re- 


380 


ELECTRICAL   CATECHISM. 


sistance,  as  with  continuous  currents.  The  ohmic  drop,  sometimes 
called  the  "effective"  or  "active"  E.M.F.,  is  at  all  times  proportional 
to  the  current  and  is  therefore  in  phase  with  it.  As  shown  in  No. 
1494,  the  E.M.F.  of  self-inductance  is  90  degrees  behind  the  current. 
These  are  the  two  components  into  which  the  impressed  E.M.F.  may 
be  resolved,  and  from  which  the  angle  between  the  current  and  im- 
pressed E.M.F.  may  be  determined.  For  example,  suppose  a  coil  has 


50 


FIG.  1497.— E.M.F.— DIAGRAM. 

a  resistance  of  10  ohms  and  an  E.M.F.  of  self-inductance  of  86.5 
volts  when  the  current  is  5  amps.  The  ohmic  drop  is  10  multiplied 
by  5,  or  50  volts.  To  find  the  impressed  E.M.F.  necessary  to  send 
the  current  of  5  amps.,  lay  off  on  a  horizontal  line  a  distance  to  repre- 
sent 50  volts  to  any  convenient  scale,  as  in  the  figure.  At  the  right 
end  of  this  line  erect  a  perpendicular  of  length  86.5  to  represent  the 
E.M.F.  of  self-induction  90  degrees  ahead  of  the  current  and  ohmic 
drop.  The  line  joining  the  ends  of  the  two  lines  represents  in  direc- 
tion and  amount  the  impressed  E.M.F.  necessary  to  send  the  current 
through  the  circuit.  From  the  figure,  this  is  seen  to  be  100  volts. 
The  angle  between  the  impressed  E.M.F.  and  the  ohmic  drop,  and 
therefore  the  current,  is  seen  to  be  60  degrees.  Knowing  the  angles 
between  the  various  E.M.F's  and  the  current,  it  would  be  easy  to  plot 
curves  similar  to  Figs.  1496  and  1499  for  this  case. 

1498.     How  is  a  sine  curve  constructed? 

At  one  end  of  a  straight  line,  A"  X,  draw  a  circle  with  a  radius 
equal  to  the  highest  ordinate  of  the  proposed  curve ;  divide  the  circle 
into  quadrants  (quarters)  by  drawing  the  vertical  line,  90-270;  by 
striking  short  arcs,  a  and  b,  from  90  to  180,  and  drawing  a  line  from 
their  intersection  through  the  center,  c,  the  circle  is  divided  into  two 
further  parts  at  135  and  315  degrees;  by  striking  other  arcs  from 


ALTERNATING  CURRENTS. 


381 


135,  cutting  the  arcs  from  180  and  90,  and  drawing  lines  from  these 
intersections,  the  circle  is  divided  into  smaller  parts  ;.this  process  may 
be  continued  as  far  as  desired.  On  the  line.  X  X,  lay  off  as  many 
points  an  equal  distance  apart  as  there  are  divisions  in  the  circle, 
making  the  distance  between  the  points  such  that  the  total  distance, 


!270° 

FIG.   1498.-CONSTRUCTION   OF   SINE   CURVE. 

o  to  360,  will  be  the  desired  length  of  the  curve ;  number  these  points 
to  correspond  with  the  divisions  of  the  circle,  and  draw  vertical  lines 
at  each  division.  Draw  lines  across  from  the  intersection  points  on 
the  circle  until  they  cross  the  corresponding  vertical  lines  and  draw 
the  curve  through  the  points  thus  found. 

1499.     How  can  the  angle  of  lag  be  determined  experiment  ally? 

The  angle  of  lag  can  be  calculated  if  the  amperes,  volts  and  watts 
are  known ;  for  the  watts  equal  the  product  of  amperes  by  volts  by 
cosine  of  the  angle  of  lag.  (The  mathematical  proof  of  this  is  some- 
what complicated  and  will  not  be  given  here).  Dividing  the  watts 


FIG.  1499.— CURRENT  LAGGING  BEHIND  E.M.F. 

by  the  product  of  amperes  by  volts  gives  the  cosine  of  the  angle  of 
lag,  from  which  the  angle  itself  can  be  found  by  referring  to  a  table 
of  sines  and  cosines.  For  example,  a  small  induction  motor  is  found 


382  ELECTRICAL  CATECHISM. 

to  take  400  watts  at  104  volts  and  6  amps.  Dividing  400  by  the 
product  of  104  by  6,  or  by  624,  gives  0.641  as  the  cosine  of  the  angle 
of  lag.  Referring  to  a  table,  we  find  that  0.641  is  the  cosine  of  50°  8'. 
Fig.  1499  shows  the  relation  of  current  and  voltage  for  this  case,  the 
current  being  plotted  to  a  larger  scale  than  the  voltage  in  order  to 
make  the  two  more  nearly  the  same  size.  In  the  upper  part  of  the 
figure,  the  two  curves  show  how  the  voltage  and  current  each  rise 
and  fall  following  "sine  curves,"  the  maximum  and  zero  values  of  the 
current  being  50°  8'  behind  the  corresponding  values  of  the  E.M.F. 
The  lower  part  of  the  figure  shows  the  same  thing  in  a  different  way, 
showing  directly  the  angle  between  corresponding  values. 

1500.  When  an  alternating  current  lags  behind  the  E.M.F. ,  is 
there  not  a  considerable  current  at  times  when  the  E.M.F.  is  zero? 

Yes.  The  current  may  be  quite  large  when  the  E.M.F.  on  the  line 
is  zero.  This  does  not  mean  that  one  could  get  current  from  a  line 
that  showed  no  E.M.F.  when  tested  with  a  suitable  voltmeter,  for 
no  current  would  flow  under  such  conditions.  It  is  true,  however, 
when  an  alternating  E.M.F.  causes  current  to  flow,  that  the  E.M.F. 
varies  from  zero  to  maximum  values  many  times  each  second,  and 
that  the  current  may  be  quite  large  at  an  instant  when  the  E.M.F.  is 
zero. 

1501.  Does  not  Ohm's  law  hold  true  for  alternating  currents? 
Yes,  it  does  when  properly  interpreted  and  applied.    It  holds  true 

for  instantaneous  values,  the  current  at  any  instant  equaling  the 
pressure  at  that  instant  divided  by  the  equivalent  resistance.  Taking 
mean  values,  impedance  is  to  be  substituted  for  resistance. 

1502.  Then  how  can  there  be  current  Homing  at  an  instant  when 
the  E.M.F.  is  zero? 

The  current  at  any  instant  is  governed  not  alone  by  the  E.M.F.  on 
the  line,  but  by  the  resultant  of  all  thcE.M.F's  acting.  A  voltmeter 
connected  across  the  line  measures  the  '  .ine"  or  "impressed"  E.M.F., 
which  is  nearly  equal  to  that  given  by  the  alternator  or  transformer  ; 
there  is  generally  in  alternating  ci.cutU  another  E.M.F.  due  to  self- 
inductance  on  the  line  or  in  the  apparatus  connected  with-it.  The 
current  in  the  circuit  is  governed  by  the  resultant  of  the  two  or  more 
E.M.F's  acting,  and  not  by  the  line  E.M.F.  alone.  The  other  E.M.F's 
are  practically  never  in  exact  step  (that  is,  In  the  same  phase)  with 
the  line  E.M.F.,  and  therefore  the  resultant  E.M.F.  may  have  a  con- 
siderable value  at  an  instant  when  the  line  E.M.F.  is  zero.  For  ex- 
ample, suppose  that  the  lines  in  Fig.  I5O2A  marked  "  Line  E.M.F." 
and  "  C.E.M.F."  represent  to  any  convenient  scale  the  mean  values 


ALTERNATING  CURRENTS. 


383 


of  the  E.M.F.  on  the  line,  and  the  E.M.F.  due  to  self-inductance 
(sometimes  called  the  counter-electromotive  force,  or  C.E.M.F.), 
and  let  the  angle  between  them  represent  the  angle  between  cor- 
responding maximum  values  of  these  quantities.  These  two  E.M.F's 


FIG.  1502A.— COMPOSITION  OF  E.M.F.'S. 

may  be  combined  and  their  resultant  found  by  the  method  explained 
in  No.  1486.  The  resultant  of  the  two  E.M.F's  gives  what  is  some- 
times called  the  "active"  E.M.F.  which  is  effective  in  causing  the  cur- 
rent. The  relations  between  the  instantaneous  values  of  the  two 
E.M.F's  and  the  resulting  current  are  shown  in  Fig.  15028,  the  cur- 


FIG.  1502B.-INSTANTANEOUS  RELATIONS  OF  CURRENT  AND  E.M.F. 

rent  at  any  instant  being  due  to  the  algebraic  sum  of  the  two  E.M.F's 
at  that  instant,  values  above  the  horizontal  base  line  being  considered 
as  positive  and  those  below  as  negative.  The  above  example  is  taken 
as  a  comparatively  simple  case,  there  being  frequently  more  than  two 
E.M.F's  in  practice.  This  example  shows  in  an  elementary  way  how 
the  current  may  have  a  considerable  value  at  an  instant  when  the 


384  ELECTRICAL   CATECHISM. 

E.M.F.  on  the  line  is  zero.  Fig.  15026  is  drawn  to  represent  a  case 
in  which  the  line  E.M.F.  is  100,  E.M.F.  of  self-induction  is  60  and 
current  (or  active  E.M.F.  causing  current)  is  80.  The  current  lags 
behind  the  E.M.F.  by  about  34  degrees  and  the  E.M.F.  of  self-induc- 
tion is  90  degrees  further  behind.  When  the  two  E.M.F's  are  equal 
as  at  A  A  in  the  figure,  the  resultant  E.M.F.  is  zero  and  the  current 
is  also  zero.  When  the  E.M.F.  of  self-induction  is  zero,  as  at  B  B, 
the  resultant  E.M.F.  equals  the  line  current  and  the  current  is  caused 
by  the  line  E.M.F.  alone.  When  the  line  E.M.F.  is  zero,  as  at  D  D, 
the  resultant  E.M.F.  equals  the  E.M.F.  of  self-induction  and  the  cur- 
rent is  caused  by  it  alone.  When  both  E.M.F's  are  in  the  same  direc- 
tion, as  at  C  C,  the  resultant  equals  their  sum. 

1503.  How  can  the  C. E.M.F.  or  E.M.F.   of  self-induction  be 
measured? 

It  can  not  be  measured,  but  may  be  calculated  from  the  triangle 
of  E.M.F's  when  the  line  E.M.F.,  current,  resistance,  frequency,  lag 
and  coefficient  of  self-induction  are  known.  In  simple  cases,  only  the 
line  E.M.F.,  current  and  angle  of  lag  need  be  known. 

1504.  How  can  the  shape  of  the  curves  of  current  and  of  E.M.F. 
be  measured  when  the  whole  cycle  is  completed  in  a  very  small  part 
of  a  second? 

The  curves  may  be  printed  automatically  by  means  of  photog- 
raphy with  the  use  of  galvanometers  having  very  light  sensitive  and 
rapidly  moving  parts.  The  more  tedious  method  is  to  measure 
the  instantaneous  values  directly  and  then  plot  the  curves  from  these. 

1505.  Explain  more  fully  how  the  curves  can' be  made  to  print 
themselves? 

A  galvanometer  is  required,  having  a  very  light  moving  system 
in  a  very  strong  magnetic  field,  as  suggested  by  the  diamond-shaped 
needle  in  the  figure.  When  no  current  flows  through  the  coils,  the 
needle  points  in  the  direction  indicated,  taking  its  position  directly 

A 


I  >  Am.Elec. 

FIG.  1505.— SECTION  OF  GALVANOMETER. 

in  the  path  of  the  magnetic  lines  of  force  between  the  poles,  N  and  5. 
Current  through  the  coils  tends  to  place  the  needle  in  the  direction 
of  the  line,  A  B,  at  right  angles  to  its  other  position.  When  the 


ALTERNATING  CURRENTS.  385 

needle  is  under  the  influence  of  both  forces  at  the  same  time,  it  will 
take  an  intermediate  position.  Now  if  the  current  varies  in  amount 
or  direction  or  both,  the  needle  will  respond  and  take  a  position  at 
every  instant  corresponding  to  the  relative  strength  of  the  current. 
A  beam  of  light  reflected  from  the  needle  or  from  a  small  mirror  at- 
tached to  it,  will  take  a  record  upon  a  photographic  plate  which  is 
moved  at  a  regular  speed  in  a  direction  perpendicular  to  the  motion 
of  the  light.  For  this  purpose  a  very  sensitive  instrument  is  re- 
quired, and  the  amateur  had  better  not  waste  time  in  trying  the  ex- 
periment. This  is  outlined  simply  to  give  a  general  idea  of  how  it  is 
done.  The  step-by-step  method  is  tedious,  but  is  delicate  to  handle. 
The  automatic  device  is  called  an  oscillograph. 

1506.  What  is  the  step-by-step  method  of  determining  the  in- 
stantaneous  values  of  an  alternating  E.M.F.  or  current? 

There  are  a  number  of -methods,  all  being  dependent  upon  the  fact 
that  no  matter  how  rapidly  the  current  or  E.M.F.  passes  through  a 
cycle,  each  cycle  is  exactly  like  those  preceding  and  those  following. 
For  example,  if  in  the  curve  in  Fig.  1498,  the  distances  above  and 
below  the  center  line  represent  the  variation  of  an  alternating  current 
through  a  single  cycle,  the  current  will  pass  through  exactly  the  same 
set  of  values  many  times  each  second.  In  the  armature  of  an  alter- 
nator there  are  usually  as  many  coils  as  there  are  poles  in  the  field. 
These  are  connected  in  series  and  are  placed  symmetrically  with 
reference  to  the  magnet  poles,  so  that  the  E.M.F's  in  the  various 
coils  are  all  in  the  same  direction  and  equal  at  any  instant ;  that  is, 
so  that  the  E.M.F.  in  each  coil  is  zero  at  the  same  time.  So  long  as 
the  load  and  speed  are  steady,  the  E.M.F.  has  a  definite  value  for  a 
given  position  of  the  armature ;  hence  if  contact  can  be  made  with  the 
circuit  for  a  very  short  time  at  some  definite  part  of  the  revolution, 
the  pressure  may  be  measured  by  a  suitable  instrument. 

1507.  What  apparatus  is  necessary  for  measuring  the  instan- 
taneous values  of  E.M.F.? 

It  is  necessary  to  have  some  sort  of  contact  device  by  which  con- 
nection may  be  made  with  the  circuit  at  any  desired  position  of  the 
armature,  also  an  electrostatic  voltmeter  or  a  condenser  and  gal- 
vanometer for  measuring  the  E.M.F.  Suppose  that  an  insulated 
metallic  disc  is  mounted  upon  the  armature  shaft  so  that  a  brush  can 
make  continuous  contact  with  it,  and  having  a  projection  from  one 
side  so  as  to  make  contact  with  a  wiper  or  spring  once  each  revolu- 
tion. It  is  plain  that  the  wiper  will  make  contact  with  the  projection 
from  the  disc  at  the  same  point  in  the  revolution  of  the  armature,  no 
matter  how  rapidly  the  armature  may  be  rotating.  Now  suppose 


386 


ELECTRICAL   CATECHISM. 


that  a  sensitive  voltmeter  is  connected  on  one  side  to  one  of  the  mains 
carrying  current  from  the  machine,  and  on  the  other  side  to  the 
brush  making  continuous  contact  with  the  insulated  disc,  and  that 
the  wiper  brush  is  connected  to  the  other  main,  as  suggested  in  Fig. 
i5<D7a.  The  wiping  brush  completes  the  circuit  through  the  volt- 
meter once  every  revolution  of  the  armature  and  at  identically  the 
same  point  in  the  revolution.  The  voltmeter  will  therefore  receive 


VOLTftETER 

FIG.  1507A.— INSTANTANEOUS  CONTACT  DEVICE. 

current  from  the  mains  for  an  instant,  and  the  needle  or  pointer  will 
be  deflected,  a  certain  amount  depending  upon  the  voltage  between 
the  mains  at  the  instant  the  contact  is  made.  By  moving  the  wiper 
to  other  positions,  the  corresponding  voltages  may  be  determined 


FIG.  1507s.— PORTABLE  CONTACT  MAKER. 

for  various  positions.  By  properly  calibrating  the  voltmeter,  the 
actual  voltage  between  the  two  mains  at  the  instant  of  contact  may 
be  determined  for  each  position,  and  a  curve  may  be  plotted,  using 
the  successive  positions  of  the  wiper  as  horizontal  distances,  and 
the  corresponding  voltages  as  the  vertical  distances.  There  are  many 
details  to  be  looked  after  for  accurate  results  and  many  different  ar- 
rangements of  the  apparatus  have  been  used,  such  as  illustrated  in 
Fig.  I507b,  but  they  are  all  essentially  the  same  as  outlined  above. 

1508.  How  can  the  curve  for  the  alternating  current  be  obtained 
by  such  a  device? 

A  small  non-inductive  resistance  may  be  placed  in  series  with  one 
of  the  mains  so  as  to  carry  the  current  to  be  measured.  By  Ohm's 


ALTERNATING  CURRENTS. 


387 


law,  the  fall  of  potential  through  the  resistance  is  directly  propor- 
tional to  the  current.  Pressure  wires  from  the  ends  of  the  resist- 
ance to  the  contact  device  and  voltmeter  as  before,  enable  us  to 
measure  the  drop  through  the  resistance  at  different  points,  and  so 
to  calculate  the  current  and  to  plot  a  curve  for  the  current  in  the  same 
way  as  was  done  for  the  voltage  between  the  mains.  For  measuring 
the  current  by  this  method,  it  is  necessary  to  have  the  voltmeter  sensi- 
tive, since  the  drop  in  voltage  through  the  resistance  will  not  be  large. 

1509.  How  can  the  relative  positions  or  the  difference  of  phase 
between  the  current  and  voltage  be  determined? 

By  having  suitable  switches,  the  wires  to  the  contact  device  may  be 
connected  first  to  one  circuit  and  then  to  the  other,  so  as  to  determine 
the  current  and  then  the  voltage  corresponding  to  each  position  be- 
fore the  wiper  is  moved  to  a  new  position.  In  this  way  correspond- 
ing values  for  primary  current,  primary  voltage,  secondary  current 
and  secondary  voltage  may  be  determined. 

1510.  What  is  a  rotary  converter? 

A  rotary  converter  is  a  machine  for  changing  an  alternating  cur- 
rent into  a  continuous  current,  or  vice  versa.  It  is  practically  a 


FIG.   1510.— A   ROTARY   CONVERTER. 


continuous-current  motor  with  a  set  of  collecting  rings  at  one  end 
like  the  rings  of  an  alternator.    If  continuous  current  is  supplied  at 


388  ELECTRICAL   CATECHISM. 

the  commutator,  alternating  current  may  be  taken  from  the  collect- 
ing rings.  On  the  other  hand,  if  the  machine  is  supplied  with  alter- 
nating current  at  the  rings,  continuous  current  may  be  taken  from 
the  commutator.  The  machine  is  often  supplied  with  a  pulley  so 
that  it  can  be  used  as  a  motor  to  furnish  power,  or  may  be  driven  as 
a  dynamo  to  generate  either  continuous  or  alternating  current.  Such 
a  machine  is  illustrated  in  Fig.  1510,  in  which  the  commutator  for 
continuous  current  is  seen  between  the  field  magnet  and  the  bearing 
at  the  right,  while  the  three  collecting  rings  for  three-phase  alternat- 
ing current  are  seen  at  the  left  between  the  bearings  and  the  field 
magnet. 

1511.  What  is  the  difference  between  a  rotary  transformer  and  a 
rotary  converter? 

These  are  simply  two  names  for  the  same  thing.  They  are  often 
called  simply  "'rotaries."  The  better  term  "  converter  "  or  "  syn- 
chronous converter"  is  recommended  by  the  American  Institute  of 
Electrical  Engineers. 

1512.  Explain  how  a  synchronous  converter  changes  current  from 
alternating  to  continuous. 

Let  Fig.  1512  represent  a  simple  ring  armature  revolving  between 
a  pair  of  magnetic  poles;  suppose  the  ring  is  wound  with  wire,  as 
shown,  two  opposite  points  being  connected  with  collecting  rings,  C 
and  C,  upon  which  brushes  make  contact  to  cake  off  the  current ;  for 
the  present  neglect  the  commutator  and  the  two  brushes  shown  in 
heavy  lines.  When  the  armature  is  in  the  position  shown,  all  the 
wires  on  each  half  are  sending  current  in  the  same  direction,  say,  up, 
making  the  outer  ring  positive  and  the  inner  ring  negative.  Now 


FIG.  1512.— DIAGRAM  OF  ROTARY  TRANSFORMER. 

suppose  the  armature  has  revolved  a  quarter  turn  so  that  the  connect- 
ing wires  are  horizontal  instead  of  vertical,  as  in  the  figure ;  then,  as 
before,  all  the  wires  on  each  side  are  trying  to  send  current  up  toward 
the  top,  but  there  is  no  connection  at  the  top  to  take  it  off;  the 


ALTERNATING  CURRENTS.  389 

E.M.F's  on  the  two  sides  of  each  half  are  opposed  and  exactly  equal, 
so  that  there  is  no  net  E.M.F.  between  the  collecting  rings,  and  there 
is  no  current  at  that  instant.  Between  these  two  positions,  the  cur- 
rent and  E.M.F.  vary  more  or  less  uniformly,  each  following  closely 
the  sine  curve  described  in  No.  1498.  Next  suppose  that  the  arma- 
ture is  provided  with  a  commutator,  each  section  of  the  winding  be- 
ing connected  with  a  commutator  bar,  as  suggested  by  the  circle  of 
heavy  lines ;  the  brushes,  B  and  B,  will  make  contact  with  different 
bars  and  therefore  with  different  sections  of  the  armature  winding, 
unlike  C  and  C,  which  always  connect  with  the  same  armature  sec- 
tion. It  is  seen  that  the  armature  wires  between  B  and  B  all  tend 
to  send  current  in  the  same  direction  at  all  positions  of  the  armature, 
so  that  a  continuous  current  may  be  taken  from  B  and  B,  while  at 
the  same  time  an  alternating  current  may  be  taken  from  the  same 
armature  by  C  and  C.  Since  either  a  continuous  or  an  alternating 
current  may  be  obtained  from  the  same  armature,  it  makes  no  differ- 
ence what  causes  the  armature  to  rotate,  whether  mechanical  power 
applied  at  the  pulley  or  electrical  power  applied  at  either  pair  of 
brushes.  As  was  explained  in  Nos.  1336  and  1346,  under  Direct- 
Current  Motors,  the  same  armature  is  equally  adapted  for  use  as 
a  dynamo  or  as  a  motor.  Therefore,  if  continuous  current  be  ap- 
plied to  the  brushes,  B  and  B,  alternating  current  may  be  taken  from 
the  brushes,  C  and  C.  Similarly,  if  alternating  current  be  supplied 
through  the  brushes  C  C,  the  machine  will  run  as  an  alternating- 
current  motor,  and  direct  current  may  be  taken  from  the  brushes 
BB;  when  driven  by  direct  current,  it  is  called  an'"  inverted  rotary." 

1513.  In  a  synchronous  converter,  is  the  voltage  between  the  alter- 
nating-currtnt  brushes  the  same  as  between  the  continuous-current 
brushes ? 

No.  The  alternating  voltage  is  only  about  seven-tenths  of  the 
continuous  voltage.  By  reference  to  Fig.  1512  it  is  seen  that  the 
alternating  voltage  is  at  a  maximum  when  the  armature  coils  con- 
nected with  the  collecting  rings  are  also  connected  with  the  con- 
tinuous-current brushes.  The  maximum  value  of  the  alternating 
voltage  is  therefore  equal  to  the  steady  value  of  the  continuous  volt- 
age. But  the  effective  or  mean  value  of  the  alternating  voltage,  sup- 
posing it  to  follow  the  sine  curve,  is  only  71  per  cent  of  the  maximum. 
For  example,  if  the  continuous  voltage  is  500,  the  alternating  voltage 
is  500  X  0.71  =  355.  In  practice  the  alternating  voltage  is  some- 
what less  than  the  simple  theory  calls  for,  being  influenced  by  the 
setting  of  the  brushes,  the  distribution  of  the  magnetic  field  and 
some  complex  elements.  (See  also  No.  1517.) 


390  ELECTRICAL  CATECHISM. 

1514.  What  kind  of  alternating  current  is  given  by  a  synchronous 
converter,  such  as  described  in  1512? 

Such  a  rotary  with  only  two  collecting  rings  would  give  a  single 
phase  or  monophase  current. 

1515.  How  could  a  two- phase  current  be  obtained? 
Two-phase  currents  could  be  obtained  by  having  a  second  pair  of 

collecting  rings  connected  to  the  armature  winding  at  points  midway 
between  the  first  ones.  The  E.M.F.  between  the  second  pair  would 
be  at  a  maximum  when  that  between  the  first  pair  was  at  zero.  The 
two  currents  would  thus  be  at  right  angles,  or  in  other  words,  in 
quadrature,  or  90  degrees  apart.  The  effective  values  of  the  two 
E.M.F's  would  be  equal. 

1516.  How  can  three-phase  current  be  obtained  from  a  converter? 
By  connecting  three  collecting  rings  to  three  equidistant  points, 

as  suggested  in  Fig.  1516. 


FIG.  1516.— DIAGRAM  OF  THREE-PHASE  ROTARY  CONVERTER. 

1517.  What  is  the  relation  betzveen  continuous  and  alternating 
voltages  in  a  three-phase  rotary  converter? 

According  to  the  elementary  theory,  the  effective  voltage  between 
any  two  of  the  three  lines  is  62  per  cent  of  the  continuous  voltage. 
Actually  it  is  somewhat  less,  for  reasons  similar  to  those  mentioned 
in  No.  1513. 

1518.  Will  a  rotary  converter  also  obtain  a  continuous  current 
from  an  alternating  current? 

Yes.  If  the  motor  is  arranged  so  as  to  be  driven  by  alternating 
current  supplied  through  the  collecting  rings,  then  a  continuous  cur- 
rent can  be  taken  from  the  brushes,  A  B,  bearing  on  the  commutator. 

1519.  Are  rotary  converters  used  much? 

They  are  largely  used  in  railway  and  other  sub-stations  for  ob- 
taining continuous  current  from  an  alternating  plant. 

1520.  What  is  a  static  transformer? 

Static  transformer  is  a  trade  or  shop  name  used  for  the  ordinary 
transformer  for  stepping  voltages  up  or  down,  to  distinguish  it  from 
the  rotary  transformer,  which  is  used  to  convert  alternating  into 
continuous  currents,  or  vice  versa. 


APPENDIX    I. 


OUTLINE   OF   ELECTRICAL   CHRONOLOGY. 

Condensed  principally  from  Benjamin,  "  The  Intellectual  Rise  in 
Electricity,"  Appleton,  1895  ;  Mottelay,  "  Chronological  History  of 
Electricity  and  Magnetism,"  Electrical  World,  1892-3;  Fahie,  "  His- 
tory of  Electric  Telegraph,"  Spon,  1884;  Fahie,  "  History  of  Wire- 
less Telegraphy,"  Blackwood,  1900;  Cajori,  "  History  of  Physics," 
S.  P.  Thompson,  "  Dynamo  Electric  Machinery,"  and  "  Elementary 
Lessons  " ;  which  writers  give  detailed  references  to  original  sources. 

B.C. 

2637  Hoang  Ti  uses  compass  in  China  (?). 
2000  Copper  plated  (?)  clay  cylinders  in  Egypt, 
looo  Lightning  protection  on  Solomon's  temple. 

600  Thales  notes  electrostatic  attraction  to  amber  (electron). 

600  Etruscans  draw  lightning  from  clouds  by  metal  arrows  (?). 

425  Euripides  writes  of  attraction  of  iron  to  lodestone. 

341  Aristotle  writes  of  the  electric  fish  (torpedo). 

A.D. 
77  Pliny  writes  of  thermoelectric  crystals  and  of  St.  Elmo's  fire. 

425  Zosimus  notes  coating  of  iron  with  copper  by  immersion. 

426  St.  Augustine  distinguishes  between  magnetic  and  electric  at- 
traction. 

looo  Marine  compass  used  by  Finns. 

1269  Petrus  Peregrinus  describes  magnetic  laws  and  a  magnetic 
motor. 

1544  Hartmann  notes  dip  of  compass. 

1600  Gilbert  writes  "  De  Magnete  " ;  makes  electroscope  and  mag- 
netometer. 

1620  Bacon  writes  "  Novum  Organum,"  the  first  "  Natural  Philo- 
sophy." 

1650  Von  Guericke  makes  electric  machine;  discovers  conduction 
and  electric  polarity. 

1655  Digges  (first  American  electrician)  notes  statk  sparks  from 
clothes ;  also  American  fireflies. 


392  APPENDIX. 

1675  Isaac  Newton  discovers  electrostatic  induction. 

1678  Swammerdam  notes  frog  leg  contraction  by  contact  with  silver 

and  copper. 

1729  Gray  distinguishes  between  conductors  and  insulators. 
1733  Dufay  distinguishes  between  vitreous  and  resinous  electricity. 

1745  Winckler,  Von  Kleist  and  Musschenbroeck  independently  make 
condensers. 

1746  Gralath  makes  electrometer,  first  electrical  measuring  instru- 
ment. 

1746  Maimbray  and  Nollet  note  effects  of  electrification  on  weight 
of  animals  and  plants. 

1747  Watson  uses  earth  return  circuit;  notes  relative  conductivities 
of  branched  circuits. 

1752  Sulzer  notes  taste  when  lead  and  silver  touch  tongue. 

1752  Franklin  shows  identity  of  lightning  and  electricity;  effect  of 

points ;  lightning  rod ;  positive  and  negative  electricity. 
1786  Galvani  notes  contraction  of  frog  leg  by  touching  unlike  metals. 
1790  Paetz  and  Van  Troostik  decompose  water  by  sparks. 
1792  Volta  proposes  contact  theory  of  electrification. 

1799  Volta  constructs  "  galvanic  "  pile. 

1800  Nicholson  and  Carlisle  decompose  water  by  current. 
1800  Wollaston  plates  copper  on  silver  by  electrolysis. 

1800  Davy  determines  electromotive   series  of  metals;  makes   arc 
light  with  carbon  electrodes. 

1 80 1  Gautherot  notes  polarization  and  reverse  current. 
1 80 1  Trommsdorff  burns  metal  by  current. 

1803  Ritter  notes. polarization  and  makes  secondary  cell. 
1806  Berzelius  publishes  his  electrochemical  theory. 

1806  Grotthus  publishes  theory  of  electrochemical  decomposition. 

1807  Davy  decomposes  potash  and  soda  electrolytically. 

1809  Sommering  operates  electrochemical  telegraph  1000  feet. 

1812  Zamboni  constructs  his  "  dry  pile." 

1812  Schilling  makes  rubber-covered  insulated  wire. 

1815  Dessaignes  finds  thermoelectric  currents. 

1815  Wollaston  makes  battery  with  large  surface  (low  internal  re- 
sistance). 

1820  Sweigger  constructs  "  multiplier,"  the  first  galvanometer. 

1820  Oersted  discovers  electromagnetic  action  of  current. 

1820  Ampere  discovers  equivalence  of  electric  current  and  magnetic 
shell;  electrodynamic  theory. 

1820  Arago  discovers  electromagnetic  rotation  (eddy  currents). 


APPENDIX.  393 

1821  Faraday  obtains  rotation  of  conductor  about  magnet  pole. 
1823  Barlow  rotating  disk  motor. 

1825  Sturgeon  makes  electromagnet. 

1826  PoggendorfT  applies  mirror  to  magnetometer. 

1827  Ohm  publishes  law  of  electric  circuit. 

1829  Poisson  proposes  absolute  values  for  magnetic  field. 

1829-30  Henry  distinguishes  between  "  quantity  "  and  "  intensity  " 
winding  of  electromagnet ;  invents  bobbin  or  spool  winding. 

1831  Faraday  finds  E.M.F.  induced  in  coil  by  permanent  or  electro- 
magnet; constructs  disk  dynamo  (unipolar  and  homopolar). 

1831  Henry  constructs  electric  motor  with  commutator;  sends  sig- 
nals electromagnetically. 

1832  Henry  discovers  self-induction  and  mutual  induction. 
1832  Dal  Negro  makes  an  oscillating  A.C.  generator. 

1832  Pixii  makes  rotating  dynamos  without  and  with  commutators. 

1833  Lenz  announces  law  of  direction  of  induced  currents. 
1833-8  Gauss  and  Weber  telegraph  with  mirror  galvanometer. 

1834  Faraday  discovers  law  of  electrochemical  equivalents. 

1834  Gauss  and  Humboldt  propose  German  magnetic  units. 

1835  Davenport  operates  miniature  electric  railway  in  New  England. 

1836  Daniell  constructs  constant-current  battery. 

1837  Steinheil  uses  earth  return  on  telegraph  system. 

1837  Morse  constructs  electromagnetic  dot-and-dash  telegraph  re- 
corder. 
1837  Sturgeon  shuttle-coil  armature  and  two-part  commutator. 

1839  Jacobi  discovers  electrotyping. 

1840  Grove  makes  incandescent  lamp  with  platinum  filament. 

1840  Pinkhus  patents  supplying  moving  car  from  stationary  con- 
ductors. 

1841  Wheatstone  dynamo  with  multiple-coil  armature  giving  con- 
tinuous current. 

1841  Deleuil  and  Archereau  arc  light  with  enclosed  carbons. 

1842  Grove  makes  gas  battery. 

1842  Henry  discovers  oscillatory  character  of  Leyden  jar  discharge. 

1842  Masson  and  Breguet  make  Ruhmkorff  induction  coil. 

1845  Wheatstone  and  Cooke  use  electromagnets  for  dynamo  fields. 

1845  Faraday  discovers  electromagnetic  property  of  light. 

1846  Bain  automatic  telegraph  with  chemical  receiver. 
1846  Gauss  and  Weber  propose  absolute  units  of  resistance. 
1846  First  paid  use  of  electric  light  in  "  The  Prophet,"  an  opera. 
1840  Mirand  invents  electric  bell  (vibrating  or  "trembling"). 


394  APPENDIX. 

1850  Page  operates  10  hp  electric  automobile  in  Washington. 
1850  Jacobi  finds  dynamo  and  motor  interchangeable. 
1855  Bessolo  patents  use  of  third  rail  for  electric  supply  (telegraph). 
1857-8  Cyrus  Field  lays  first  transatlantic  cables. 

1859  Farmer  lights  house  by  platinum  incandescent  lamps. 

1860  Plante  makes  storage  battery. 

1861  British  Association  adopts  "  B.A."  unit  of  resistance. 

1862  Arc  light  in  Dungeness  lighthouse,  Holmes  alternator. 

1864  Maxwell  shows  mathematical  equivalence  of  light  and  elec- 
tricity. 

1866  First  successful  transatlantic  cable. 

1867  Werner  Siemens  makes  self -exciting  dynamo. 

1867  Pacinotti  makes  toothed  armature  (open  coil  winding). 

1868  Stearns  duplex  telegraph. 

1871  Gramme  ring  armature  with  continuous  circuit. 
1871  Hefner  Alteneck  drum-wound  armature. 
1873-4  Heaviside  and  Edison  invent  quadruplex  telegraph. 
1873  Rowland  law  of  magnetic  circuit  analogous  to  Ohm's  law. 
1873  Fontaine  and  Gramme  transmit  power  from  generator  to  mag- 
netic motor  at  Vienna  Exposition. 
1876  Bell  exhibits  telephone  at  Philadelphia  Centennial  Exposition. 

1876  Brush  operates  arc  lamps  in  series. 

1877  Jablochkoff  uses  transformers  in  series  on  A.C.  circuit. 

1878  Hughes  invents  microphone. 

1878  Avenue  de  1'Opera  in  Paris  lighted  by  arc  lamps. 

1879-81  Brush,  Metzger  and  Faure  make  "  pasted "  storage  bat- 
teries. 

1879  Siemens  &  Halske  third  rail  electric  railway  at  Berlin  Ex- 
position. 

1879  Edison  makes  incandescent  lamps  with  high  resistance  carbon 
filament  in  vacuum. 

1879  California  Electric  Light  Co.  incorporated  for  series  arc  light- 
ing. 

1 88 1  Joubert  founds  modern  theory  of  A.C.  machines. 

1881   International  Congress  adopts"  C.G.S."  units. 

1 88 1  First  commercial  electric  railway  in  Berlin. 

1882  Electric  power  transmitted  37  miles  to  Munich  Exposition, 
i  hp,  38  per  cent  efficiency. 

1882  Edison  station  in  New  York  for  incandescent  lighting  at  con- 
stant potential. 

1883  Car  operated  in  Paris  by  storage  batteries. 


APPENDIX.  395 

1883  Gaulard  and  Gibbs  use  transformer  for  constant  potential 
service. 

1883  Hopkinson  improves  magnetic  circuit  of  dynamo. 

1884  Bentley-Knight  conduit  railway  in  Cleveland. 

1885  Henry  builds   electric   railway   with   overhead   conductors   in 
Kansas. 

1885  Ferraris  discovers  rotary  field. 

1886  Cowles  produces  aluminum  alloys  in  electric  furnace. 

1886  Stanley  distributes  alternating  current  with  step- down  trans- 
formers in  multiple. 

1888  Tesla  constructs  rotary  field  alternating  current  motor. 

1888  Hertz  demonstrates  electric  waves. 

1888  Sprague  constructs  first  modern  electric  railway  at  Richmond. 

1889  Hall  produces  pure  aluminum  in  electrolytic  furnace. 

1890  Branly  discovers  coherer. 

1891  Borchers  produces  calcium  carbide  in  electric  furnace. 

1891  Courts  decide  telephone  interests  do  not  own  the  earth  as  re- 
turn circuit. 

1891  Power  transmitted  no  miles  LaufTen  to  Frankfurt,  three- 
phase,  16,000  volts,  200  hp. 

1893  International  units  adopted,  including  watt,  joule  and  henry. 

1893  Howard  and  Marks  introduce  enclosed  arc  lamp. 

1893  Gray  invents  autographic  writing  telegraph. 

1894  Acheson  produces  carborundum  in  electric  furnace. 

1895  Electric   power  transmitted   from   Niagara   Falls. 
1895  Rontgen  discovers  X-rays. 

1895  Electric  locomotives  supplant  steam  locomotives  at  Baltimore. 

1895  Three-phase  A.C.  railway  opened  at  Lugano,  Italy. 

1896  Acheson  produces  graphite  in  electric  furnace. 

1897  Marconi  sends  aerial  messages  nine  miles. 

1901  Hewitt  mercury  vapor  arc  light.     Mercury  vapor  A.C.  rectifier. 
1901  Nernst  lamp  with  earthy  conductors. 

1901  New  York  Central  R.   R.  prepares  for  electrification. 

1902  Lamme  series  alternating  current  railway  motor. 

1902  Birkeland  and  Eyde  fix  atmospheric  nitrogen  commercially. 
1905  Tantalum  high-efficiency  incandescent  lamp  introduced. 

1905  Osmium,  wolfram  and  metalized  carbon  incandescent  lamps. 

1906  Osram  and  tungsten  incandescent  lamps. 

1906  New  York  Central  electric  locomotives  in  service.   ' 

1907  Transatlantic  radio-telegraphy  established. 


UNIT    EQUIVALENTS. 


(Compiled  from  Bering's  "Conversion  Tables.") 
LENGTH 

i  mil  =  o.ooi  inch 0.02540005  millimeter 

i  inch 2.540005  centimeters 

i  foot  (U.  S.)    30.4801  centimeters 

i  mile 1.60935  kilometers 

i  centimeter   (cm.) °-3937  mcn 

i   kilometer   3280.83  feet 

AREA 
i  circular  mil  (CM) 0.785398  square  mil 

0.000506709  square  millimeter 
i  square  inch I  273-240.  circular  mils 

645.163  square  millimeters 
i  square  centimeter 197  352.  circular  mils 

0.155  square  inch 

VOLUME 

i  cubic  inch 16.38716  cubic  centimeters 

o.ooo  578704  cubic  foot 

i  cubic  foot 28  317.  cubic  centimeters 

7.48052  gallons  (liquid  U.  S.) 
6.42851  gallons  (dry  U.  S.) 
"  0.037037  cubic  yard 

i  cubic  yard   46  656.  cubic  inches 

0.764  559  cubic  meter 

I  pint  (liquid  U.  S.)   473.179  cubic  centimeters 

28.875  cubic  inches 
16.  fluid  ounces 
0.859367  dry  pint 

I  gallon  (liquid  U.  S.)   ...  3785.43  cubic  centimeters 

231.  cqbic  inches 
0.133681  cubic  foot 

i    gallon    (British    or    Im- 
perial)     4545.9631  cubic  millimeters 


398  APPENDIX. 

i  cubic  centimeter 0.0610234  cubic  inch 

0.001  liter 

i  liter 61.0234  cubic  inches 

2.11336  pints  (liquid  U.  S.) 
1.81616  pints  (dry  U.  S.) 

WEIGHT 

i  grain 0.064  799  gram 

0.00228571  ounce  avoirdupois 

i  ounce  (avoirdupois)  . . .  .437.5  grains 

28.3495  grams 

0.0625  pound  avoirdupois 

i  pound  (Ib.  avoirdupois)   .  7000  grains 

453.5924277  grams 

i  ton,  2000  pounds   907,185  kilograms 

i  long  ton,  2240  pounds  .  .  1016.05  kilograms 

i  gram i5-43235639  grains 

0.035274  ounce  avoirdupois 
.00220462  pounds  avoirdupois 

i  cubic  centimeter  water  .  .  i  gram 

i  cubic  foot  water 62.4283  pounds  avoirdupois 

28.317  kilograms 

i  gallon  water  (liquid  U.  S.)  8.34545  pounds  avoirdupois 

3.785  43  kilograms 


((  (f  U 


ENERGY       WORK       HEAT 

i  erg  or  dyne-centimeter.  .  o.ooooooi  joule 

"  0.00101979  gram-centimeter 

i  gram-centimeter   980.5966  ergs 

0.00007233  foot-pound 

i  joule,  watt-second 10  197.9  gram-centimeters 

"  0.737612  foot-pound 

"  "  0.101979  kilogram-meter 

0.00094796  thermal  unit  (B.T.U.) 
0.238  882  small  calorie 

i  calorie,  large 1000  small  calories 

4186.17  joules 
3087.77  foot-pounds 
3.968  32  British  thermal  units 
"  "  1.16282  watt-hours 

"  o.ooi  559  48  horsepower-hour 


<t 


APPENDIX.  399 

I  thermal  unit  ( British,  B.T.U.)  i  Ib.  water  i  degree  Fahrenheit 
"         "  "  "        1054.9  joules 

778.104  foot-pounds 

"         "  "  "        251.996  small  calories 

"         "  "          "        1.41474  horsepower-seconds 

"         "  "  "        o.  293  027  watt-hour 

i  foot-pound 1.355  73  joules 

"  13  825.5  gram-centimeters 

"  o.ooi  818  1 8  horsepower-second 

o.ooi  285  17  thermal  unit  (B.T.U.) 
"  0.323  859  small  calorie 

"  0.000000376591  kilowatt-hour 

i  horsepower-second 745-65  joules  or  watt-seconds 

550.  foot-pounds 
"  178.122  small  calories 

76.0404  kilogram-meters 

i  horsepower-hour 2  684  340.  joules 

"  745.6"5  watt-hours 

1  980  ooo  foot-pounds 
641.24  large  calories 

1.01387  metric  horsepower-hours 
"  2544.65  thermal  units  (B.T.U.) 

i  kilowatt-hour 3  600  ooo.  joules,  watt-seconds 

2  655  403.  foot-pounds 
367  123  kilogram-meters 
3412.66  thermal  units  (B.T.U.) 
i  .34 1 1 1  horsepower-hours 
1.35972  metric  horsepower-hours 

POWER,       RATE  OF  DOING  WORK 

i  watt i  joule  per  second 

10  ooo  ooo.  ergs  per  second 
10  197.9  gram-centimeters  per  second 
0.737612  foot-pound  per  second 
0.0568776.  thermal  unit  (B.T.U.)  per 

minute 
I  horsepower  (English)   ..745.65  watts 

"  550.  foot-pounds  per  second 

42.41  thermal  units  per  minute 
10.6873  large  calories  per  minute 
"  "  1.01387  metric  horsepower 


INDEX. 


(The  figures  refer  to  the  numbered  paragraphs.) 


Absolute  standards 925 

Abuse   of   instruments 903-9 12 

Accelerator'    788 

Accumulator  (see  Storage  batteries). 

Acetate  of  lead 635 

Acetylene   gas    5°7 

Acheson     4J3»  5°6 

Acids,   conductivity  of 33 

Active  E.   M.   F 1502 

Advantages  of  electrodynamometer .  . .      828 

series  motors   i324 

shunted    ammeters    841-843 

shunt  motors   1331 

Aging  a  magnet 768 

Air    33.  79.  80,123,   148-153,  745 

(See  also  Atmospheric  electricity.) 

Air  gap    1168,1169 

Alkaline    metals    649   (7) 

Alternating  current  .  101,  300,304,  374,  375 
1125,   1131,1400-1520 
conducting  systems, 

1443-1447,   1452,   1453 

frequency    1401,   1433-1436 

frequency,  determining    1435.   J436 

generators   (see  Alternators). 

generator  regulation   1462-1467 

instruments     919 

measurement    920,  934 

motors    1001-1003,   135^,   1364 

phase  difference   1442,   1493,   1502 

polyphase    1442 

quarter-phase     i437»   1442 

rectifier     1469,   1470 

transformer      497,   1404-1426 

arc  light     1425,   1426 

connections    1409-1415 

constant   current    ..1425,   1426,  1472 

current    1425 

instrument      1417 

primary  current   1420,   1424 

ratio  of  currents 1420-1426 

ratio  of   E.    M.    F.'s    (see   also 
Choke  coil), 

1406-1408,   1416-1419 

series   1425,   1470 

single-coil      1463 

two-phase     1437-1439,   1442-1448 

use    of    diagrams 1485-1499 

uses     .497,  503,  511,  1402 


Alternator    ...1114,   1121,   1125,  1138,  1256 
1400,  1427-1470 

armatures     1182,   1431,   1439,  1441,   1448 
1449    1459,   1463 

composite     1425,   1468 

field    exciter    1454-1458,   1462-1470 

General   Electric    1458 

Heisler    1467 

induction     *454 

inductor     1459-1461 

monocyclic    1 1 53 

regulation    1462-1467 

revolving   field    1458-9,   1463 

Stanley     1460,   1461 

Warren      1460 

Westinghouse   1119,   1121 

Aluminum     426,   513,   514 

Alundun    505 

Amalgam    39,  620 

Amber    2,   10 

American    Institute   of    Electrical    En- 
gineers        1511 

American    instruments    914 

Ammeter    1216,   1223,   1226,   1425 

adjustable    shunt     842,  843 

Ammeters   and   voltmeters 752 

Ammonia    from    air 154 

Ammonia   gas    615 

Ampere    137,  227,  236,  241 

Andre    227 

international      225,  227 

legal     236 

meter     844 

turn    247,  248 

turns,  calculation  of 733-737 

Ampere-hours      816,  818 

Analogy  between  water  and  electricity, 

308,  949 

Angle  of   lag 1482,   1484,   1492-1499 

Animals,  conductivity  of 33 

Anions    644,  649   (4) 

Annealing    465,  499 

Anode     607,  644,  645,  649   (7),  815 

Antimony     636 

Arc    483,  511 

alternating   current    ....511,   1477,   1483 

C-   E.  M.   F I477 

circular    1498 

deflected  by  magnet 491,  502,  511 


404 


INDEX. 


Arc  dynamos  coupled  in  series.  .1217-1221 

lamps      311,  330,  444,  468,  487-493 

H43 

alternating  current   .  .485,  486,  1477 

1483 

enclosed    ..  .484-487,  959,   1477,   ^83 

flaming     468,  486,  491,   1425 

high  tension     485.   1416 

low  tension    485 

magnetite     487.  488,  492 

mercury   vapor     468,  486,  493 

metallic  flame     487,  488,  492 

open      330,  484-487,  959 

light,    i20o-cp,    200o-cp 486,   1143 

light    circuit     932,  959,   1155,   1156 

1477,   1483 

light  dynamo    ....1117,  1142-1145,  1162 
1208,  1217-1221 

light  transformers   1425,   1426 

used  for  welding 501 

Aristotle    10 

Arithmetic  applied  to  alternating  cur- 
rents        1446 

Armature     788,   1004,   1006,  1116-1119 

1300-1305 

core    1117,   1118,  1302,   1304 

reaction     1162,   1243,   1431 

ring,  drum 1117 

Arnold     785,  788,  793 

Arsenic     505 

Asbestos     440,  449,  455,  460 

Atmospheric   electricity    ...10,  97,   108,   no 

UI-IS7 
(See  also  Air.) 

Atoms    644 

Attraction,  magnetic    712,  779-793 

1300-1303 
and  repulsion  of  charged   bodies, 

17,   19,  20-25,  74,  88,  89,  96-98 

Aurora    151,   152 

Automatic  electric  gas  burners 55,  57 

rheostat    i334-J345 

Automobiles    604,   1266 

Auxiliary  bus-bars    1262 

Avoirdupois  weights    206 

"B"    725 

Balanced  load   1452 

Ballistic    galvanometer    140,   141,  1507 

Bastian  meter   8 1 6 

Bathurst    430 

Battery     307,  601-642,  654,  iioo 

bichromate      624,  625,  630 

blue  stone    626 

Bunsen    631 

central  station    641,  642 

Clark     228 

closed   circuit    625 

crowfoot    626 

Daniell     632 

depolarized  mechanically    629 

Diamond    615 

dry     622 


Battery,     Edison-Lalande 628 

Fuller    625 

Gassner    622 

Gordon    628 

gravity     626 

Grenet    623 

Hayden     6 1 6 

Laclede    615 

Law    615 

Leclanche    615 

local   circuits    618,  619 

motors    1300-1303,   1323,   1325 

open   circuit    611-625 

Partz     624 

plunge     623 

sal  ammoniac    615-622 

Samson    6 1 6 

silver    chloride    621 

Since    629 

storage    635-642 

Bauxite     505,   513 

Becquerel    413 

Bell  Telephone  Co : . .     441 

Bells,   electric    98,  616,  786,  787,   1005 

1009-1013 

Belt,  slipping 1259 

Belts,   electricity   from 37,  59-73,  81-88 

Berliner     412 

Bernardos     496,   501,  502 

Berthelot 506 

Biased    bell    1015 

Bichromate  of  potassium 624 

Binary  compounds   649    (6) 

Bipolar  dynamo    1120,    1123-1125,    1182 

1190 

Birkeland    511 

Bismuth    426 

Blasting    fuse     444,  445 

Bleaching  by  electricity 68,  465 

Blinking   lights    816 

Blondel   oscillograph    913 

Blount     * 654 

Blowing  of  fuse  427,  433,  1215,  1236 

1343 

Blue  stone,  blue  vitriol 626,  632 

Bolometer    812,  813 

Books,   electrochemistry    654 

wiring   376 

Booster  1260,  1261,  1263,  1471,  1472 

Borax  435,  500 

Borchers     510,   515,  654 

Bottle    warmer    456 

Bradley-Lovejoy     511 

Branch  blocks   434 

Branched    circuit    331 

Branding  irons   444 

Brard  generator    413 

Brazing    502 

Bridge,    Wheatstone. 403,  449,  948-954 

Bristol  recording  voltmeter 830 

British  association    237 

unit    237 

thermal  unit,   B.   T.   U 220 


INDEX. 


405 


Brush   arc   dynamo 854>   117° 

Brushes     1114,   1117,   "62,   1188-1193 

Brush  position 1257-1260 

Brotherhood  engines    1 160 

Brown  &   Sharp  wire  gage 358,  359 

Buckling  of  battery  plates 637 

Bug  cut-out    438 

Building  up  of  dynamo 1172,   1201-1209 

Bunsen    cell    631,  632 

Burners,  automatic   55>  57 

gas     43-57 

jump  spark    43'49>54-57 

pendant   or   ratchet 54>   56 

Bus-bars     .350,  840,   1224,   1254,   1262,   1466 

Butte    H35 

Buzzer     1013 

B.  W.  G 358 


Cabinet   system   of   wiring 440 

Calcium    carbide    504-510 

Calculation    of    current    320-329 

efficiency  of  motor.  1314-1318,   1362-1367 

electromagnet     715-721,  727-739 

pull  on  wire 1306-1312 

resistance       ....323,  324,  328,  333,  334 

348 

wires     348-359,  367"374 

Calibration  of  instruments    849,  920 

924-930 

Calorie    221,  415 

Calorimeter     804 

Candle   and  static   electricity 104,   106 

Candle-power    472,  478 

of  arc  lamps 486,   1143,   1144 

Canyon   Ferry    H35 

Capacity, 

dynamo,   determining    U77,   1180 

dynamos  in  series   1218,   1219 

electrostatic     115-121,   139,   1494 

lines    (electrostatic)    ....1131-1136,   1497 
(See  also   Condenser.) 

specific    inductive    1 18-122 

Carbide     504-5 10 

Carbon   battery  electrodes 608,  614,  615 

619,  629,  631 

brushes 1 192 

conductivity    33 

filaments    for   incandescent   lamps    .  .469 

474-478 

pencils    for   arc   lamps 483-491 

telephones      346,  347 

Carbon   bisulphide    505 

Carborundum    505-508 

Cardew    voltmeter    806 

Carhart    654 

Carty,  J.  J 101 

Case  thermo   generator 413 

Cash  carriers,   magnetized 780 

Cast  iron   (see  Iron  for  magnets). 

Castors,    electric     72,   73 

Cast-steel   magnets    1320 

Cat,    electricity    from 19,  41 


Cathode     607,  644,  645,  649   (9),  649 

(n),  815 

Cations     ^.  .644,  649   (5) 

Caustic  potash   628 

soda    505,  628 

Cautery    465,  466 

Caution   (see  Danger,  Fire,  Shock), 

958,   1136,   1214,   1215,   1220,   1225 
1230,   1236,   1239,   1412,   1414,   1425 
Cell   (see  Battery). 

Celsius   thermometer    222 

C.  E.  M.  F  in  arcs 1477 

in    batteries    612,  613,  650-653 

in   choke   coils    1474-1477 

in    motors     ..1336,   I347-I35O,   1357-1373 

in  Ohm's  law 361,  365,  366 

in   transformers    1405,   1420,   1423 

on  alternating-current  lines..  1495,   1496 

1502,   1503 

Centigrade    thermometer    ....205,  222,  223 

Centimeter 200,  202-204 

Central  station  practice, 

93-95,  638,    1175-1268 

"  C.   G.   S."   system  of  units 200-209 

Chafing   dish,    electric 456 

Charging,    current     1132,   "35 

Charging  by  static  electricity 16,   19,  23 

storage   batteries    638-640 

Chemical   action    413,  600-653 

cell    (see  Battery.) 

effects  of  current.  ..  .227,  311,  317,  635 
639,  643,  649,  653 

meters     815-821 

paper    639 

work    649   (13),  650 

Chicago     International     Congress     of 

Electricians    225 

Chinese  compass   9 

Choke  coils    1425,   1473-1477 

Chromic  acid,  alum,  sulphate 624 

Cigar  lighter,  electric 133 

Circuit    breaker     422,   1215,   1425 

Circular    mils    341,   343,   349,  35»,   1180 

Clark  cell    228 

Cloud  electricity 40,   149,   150 

(See   also    Atmospheric   electricity.) 

Coercive   force    762 

Coercitive   force    762 

Coffee   pot,   electric 444,  457,  460 

Cold  and  electricity.  . 80 

Collecting  rings  ....1114,   1182,   1427,   1429 

1441,   1510 

Comb,  electricity  from 19,  25,  39 

Combination    of    currents    of    E.    M. 

F.'s    1447,   1486-1502 

Commutation     1114,   H93,   1245,   1377 

Commutator   ..1114,   1117,   1188,   1193,   1213 
1303,   1427,   1429,   1510 

Compass   ..9,  310,  704,  703,   758,  775,   "09 
1123,   1199,   1202-1204 

Compensator     *472 

Composition   of   forces    1486-1502 

Compound  magnet   77 J 


406 


INDEX. 


Compound  dynamos  ..  .1150-1153,  1244-1259 

1468-1470 

Condenser  111-127,  J34>  802,  1132 

ii33»  1507 

Conduction  of  currents    35-37 

Conductivity   of   different   substances.        33 

Conductors    and    insulators 32-35,  338 

Connecting  up  a  dynamo.  ..  1200-1203,   1212 

Consequent  poles   744 

Conservation   of   energy 213 

Constant-current  dynamos    .1140-1143,   1158 

transformers    1425,   1426,   1472 

Constant-potential  dynamos.  1146-1150,   1158 

Constitution  of  matter i,  644 

Continuous-current   motor    1510 

Converters     (see     also     A.     C.     Trans- 
former)       497,   1510-1520 

Cooper     i 654 

Cooper-Hewitt   lamp    493 

Cooper  pyro-magnetic  motor 412 

Copper    33,  426,  448,  626,  628 

brushes .    1192 

sulphate     626,   632 

Cost  gas  lighters 47-50 

of  electricity    1 104 

to  cook  by  electricity 457 

Coulomb    136-140,  229,  317 

Counter     1017" 

magnetomotive   force    1423,   1425 

electromotive     force     (see     C.     E. 

M.   F.) 
Coupling   dynamos    ...1217-1232,   1249-1259 

Cox  thermopile    404 

Cross-talk  on  telephones 100-103 

Crowfoot  battery    626 

Crown    1460 

Cryolite     513 

Curling  iron  heaters 444 

Current,    alternating    804,   1400-1520 

carrying   capacity    418,   1180 

electric    4,  6,  35,  36,   158,  309 

fusing    430 

in  arc  lamps 485 

indicator    845,   1216 

measured  by  fall  of  potential.  .922,  923 
measured  without  opening  circuit, 

921-923 

measurement     804-927 

taken  by  motor 1368,    1369 

thermo-electric    403-410 

unit    of    227,   241 

(See  also  Ampere.) 

various  kinds    300-306 

with  no  E.  M.  F 1502 

Gushing     376 

Cut-outs     424,  434,  439,  488 

Cutting  iron  beams 502 

Cycle     1433,   1434,   1478,   1480,   1506 

Dancing  dolls 88,  89 

Danger   from  electricity 62-67,   73 

I55-I57,   1220 
(See  also  Fire,   Shock.) 


Daniell   cell    632 

d'Arsonval   galvanometer    ....833-840,   1004 

Davy     483,   506 

Dead  center   1480 

De   la    Rive    635 

"  Delta  "   connection    1450 

Dentists     465 

Depolarizer     610,  613,  616,  632 

Deposition     227,  649   (14) 

Deprez  galvanometer   913 

Derivation  circuits   331 

Desiccator     465 

Designing  electromagnets    728-739 

Detachable    fuse    437 

Detection    of    static    electricity.  ...  17-20,  61 

Detector   galvanometer    1 109 

Determining   resistance  of  wire 348 

(See  also  Measurement.) 

Determining  polarity   639 

Diamond    battery    615 

Dielectric    106,   118-123 

Difference  between  ammeter  and  volt- 
meter          902 

ammeter   and   ampere-meter 844 

battery  and  dynamo  current 307 

cell   and   battery 602 

Difference     between     magnetism     and 

electromagnets     701 

permanent  and  electromagnets....      759 

positive  and  negative    26 

primary   and   secondary   battery...      605 

voltmeter    and    voltameter 846 

Difference    of   phase    1442,    1493,    1509 

potential    361 

Differential    galvanometer    813 

Diffused   discharges    35 

Dipping  needle   781 

Direct-connected    machine     1457,   1458 

Direct-current   generator    ..1100-1269,   1427 

i43i 

motor     1000-1013,   1300-1388 

Direction  of  current 309,  310,   mi 

rotation  of  dynamo  and  motor...    1231 

pull  on  a  wire 831,  832 

Disadvantages  of  electrodynamometer.     829 

series  motors   i324 

shunt    motors    1331 

Disc  heater    456 

Discharger   for  static   electricity 76-79 

81-84,   101-104,   155,   156 

Discovery  of  compass 9 

electric  current   1 1 

electric  waves    12 

static  electricity    10 

Disinfecting    by    electricity 68 

Dissipation  of  electrical  energy 446 

Distortion  of  magnetic  field 1258 

of   wave   form 1478 

Distributed  winding 1431 

District  telegraph    1008 

Divided  circuit    33* 

Dividing  load  between  dynamos 1226 

1227,  1232,   1249-1259 


INDEX. 


407 


Dolls   and   electricity 88,  89 

Dots  and  dashes 1006 

Double-current  generators 1429*   *43O 

pole  fuses   439 

switches 1 228 

scale  voltmeter    847,  907,  933 

Dressmakers  and  electricity 64,  65 

Drop    ...308,  361,  367,  368,  371,  373.   I26i 

Drum   armatures    1117.   J304 

Dry   cells    622 

Drying  by  static   electricity 68,  90 

Duluth     1417 

Dynamo    §38,   1100-1269 

as  a  motor 1231 

brushes    1189-1193 

field    1120-1125,   1239 

Dynamos,  how  E.  M.  F.  is  generated.    1114 

in   multiple    1222-1236,    1249-1259 

in   series    1220,    1221 

Dynamotor    1265-1269 

Dynamo  refusing  to  pick  up ....1185 

1194-1197,   1201-1209 

Dynamos,   source  of  voltage 41 

Dyne    208,  209,   1306-1309 

Dyne-centimeters    1309 

Early    knowledge    of    electricity    and 

magnetism    2,  9-11 

Earth  (see  Ground). 

Earth's  attraction   208 

magnetism    774 

Eccentric  instruments   855 

Eddy   currents 788,  919,   1118,   1431 

Edison     337,  412,  435,  652,  791,  792 

810,  816,  846,  854,  1120,  1162,  1308 
chemical  meter. 652,  649  (u  ),  816,  846 

dynamo  1 1 20,  1162 

•  Lalande   cell    627,  628 

toy  motor 1 302 

Effect  of  leveling  instruments 916 

Effective  magnetic  lines  of  force 1360 

resistance     330,   1501 

Effects   of   static   electricity.  .  17,   19,  97,  98 

no,   154 

Efficiency    of    motor 1363-1365 

incandescent  lamp   473-477 

storage  battery    642 

Eickemeyer    dynamo    1 160 

Electrical   measurements    900-964 

measuring  instruments   800-856 

standards    225-227,  925,  926 

Electric    arc    483-495 

bell     711,   1005,   1010 

belts     59,   70-73 

blowpipe,  Zerner   501,  502 

breeze     1 04-  no 

castors    72,   73 

corsets     783 

counter    1017 

current   (see  Current,  electric). 

fan     458 

flat-iron 456 

forging    444 


Electric    furnace    503-513,600 

glue  pot    444 

hair  brushes 783 

heat    414-513 

heater     444,  458 

heating  and  cooking  utensils 455 

heating   pads    444,  456 

laundry    456,  458 

melting   pot    444 

.ovens     444,  499,  503 

pads  and  plasters 783 

refrigeration     409 

soldering  iron   444,  455,461 

spark    ..17,   19,  42,  50-53,   131-134,   1239 
(See  also  Sparking.) 

stoves     444,  456 

tea   kettle    444,  456 

tempering     " 444 

thermometer    812 

waves    4,   7,   12,    132 

welding 444,  495-502,   1403 

whirligig    104-106 

wind    104-1 10 

Electricity    and   chemical    action.  ..  .600-653 

analogy  to  flow  of  water 308,  949 

at  rest   4,   147,   158 

denned    i  -8 

from   friction   or   rubbing..  19,  27-29,  38 
39,  42,  47,   59 
(See  also   Static  electricity.) 

in    motion     4,  6,   158 

(See  also   Current,   electric.) 

in    vibration    4,   7,   12,   132 

of    clouds    40,   148,   149,   157 

what    is    it      1-8,  644 

Electrification    16,    148,   151 

Electro-chemical  current  meter 815 

depolarizer    632 

equivalents     647,  648 

relations  of  current 600-654 

ring    783 

series    608 

Electrode    607-609,  614,  615 

arc   light    484-493 

wooden    35 

Electro   deposition    227,  643-653 

Electrodynamometer     825, -827,  920 

Electrolysis    643,  645-650 

of   pipes    649   (15) 

Electrolyte    606,  612,  614,  621 

623,  640,  645,  646 

Electrolytic  refining  of  metals 649   (8) 

(n) 

Electromagnet     contrasted     with     per- 
manent   magnet     759 

Electromagnetic     control 422,  469,  488 

1215,   1345,   1425 

design      728 

ring 783 

throttling   valve    1 160 

units     136-139,   224-227,   246 

Electromagnetism     700-793 

Electro-mechanical    bell     .  .1012 


408 


INDEX. 


Electro-medical     treatment 35,  51,  70-73 

300-306,  466 

metallurgical    processes    444,  600 

Electrometer     20-24,   142-147 

Electromotive  force    (see  E.   M.    F.). 
counter    (see  C.   E.   M.    F.). 

Electron     2,   10 

Electro-positive  and  electro-negative, 

25,  26,  29,  608,  609,  644,  649 

Electroplating      405,  629,  649 

Electroscope    20-24 

Electrostatic    (see  Capacity,   static). 

ground    detectors    801-803 

induction    40,  99-101 

(See  also  Induction,  electrostatic.) 

repulsion     1 008 

units    136,   137,  224 

voltmeter    143-147,  801-803,   1507 

Elementary    motors    824,    1000-1013 

E.    M.    F..I37,   138,   142-147,  360,  361,  623 

active     1 502 

in    arc    489,   1477 

in  field  magnet  coil 1239 

instantaneous    values    1505-1509 

measurement   of.. 319,  363,  901,  928-934 
(See  also  Voltmeter.) 

of  a  dynamo 1106-1124,   1130 

1142-1176,   1184-1268 

of  a  transformer   1405-1419 

of   an   alternator.  ..  1427-1441,   1455-1471 

Enamel    455 

Enclosed   arc   lamps.  ..  .484-489,   1477,   1483 

End  cell  switches 638,  641 

Energy     210-212,   215,  218,    1475-1477 

conservation  of   213 

of   alternating  currents 1478-1484 

lost  in  choke  coil   and  rheostat...    1475 
required  by  heating  appliances     .  .  .  .456 

457 

Engine,   gas    1135 

Engine  speed,  counting 1186,   1187 

Equalizer     1249-1254 

Equivalent    resistance    1501 

Erg   209-2 1 1 

Etard     506 

Ettinghauser    413 

Excelsior  arc  dynamo 1162 

Excessive   heating,    avoiding 421 

Exciter    1 138,   1456-1458 

Expansion   of   conductors 805-807 

Explosives     511 

Eye  magnet    790 

Factor  of  safety   731 

Fahrenheit     thermometer 205,   222,   223 

Fall    of   potential.  .  .308,  361,  922,  923,  930 

Fans,    electric    458 

Farad     136,   137,   139 

international    230 

Faraday    139,  230,  635,  646,  649 

Faraday's  laws  of  electrolysis 646 

Faradization   303 

Faradic   current    300 


Faure  secondary  battery 635 

Feather    electroscope    20,  22,  37,  99 

Feeders    1261-1263 

Fertilizer 511 

Field    magnet     732-739,   1320-1322 

magnetic     447,   702-711,   1115,   1301 

stray      745-758 

coil     733-739,  937,   "IS,   "24 

1137-1140,   1146,   1149,    1150 
Fire  alarm    443,  626,   1009 

from  electricity,  43-57,  64,  65,   133,   134 

Fixtures    43,  45,  418 

Flaming    arc^lamp 468,  486,  491,   1425 

Flashing  at  brushes 1213 

motor  switch    1333,   1338-1342 

Flat  iron,  electric 455,  458 

Fluorides 513 

Flywheel     1459,   1480 

Fog   and   static    electricity "      97 

Foot    204,  217 

-pound     210-212,  215,  220,   1311 

i3M,   1315,   1352 

-pound   second    212,  214 

Forbes  meter   808 

Force   units    208 

Forging,  electric   444 

Fort  Wayne  arc  machine 1162 

instruments    855 

Foster     515;  654 

Foucault    currents    1118 

Four-pole    dynamo    1 189 

Four-wire  circuits    1443,   1452 

Fractional  measurement  of  voltage    ....931 

93.2 

Franklin,  Benjamin       98 

Franklinization    302 

French    (see  Metric  system). 
Frequency   of   alternating   currents    ..1015, 
1016,   1401,   1433-1436 

changer   1264 

Friction    losses     1241,   1363 

Frictional   electric   machine    39-48,   59 

Friction,    electricity    by 19,  27-29,  38 

39,  42,  47,   59 

Full  arcs    1 1 43 

Fuller  cell    625 

Furnace,   electric    503-515,  600 

electric    fan    for 458 

Fuse     422,  423,465 

block     433-443 

link 424 

panel     440 

plug    424 

Fuses  in   multiple 431,  432 

Fusible    424 

Galvani  603 

Galvanic  cell  (see  Battery). 

current  300,  301 

Galvanometer  140,  141,  407,  843,  847 

1109,  1229,  1504,  1505,  1507 

Deprez  913 

Gas  battery  634,  635 


INDEX. 


409 


Gas   lighting   by   electricity 42'57»   *34 

pipe  return  45 

Gas    engine    H35 

Gassner  battery    622 

Gauss     242 

Gautherot  secondary  cell    635 

Geisler   tube    1 29,   130 

General  Electric  Company.  ..  .476,  803,  855 
1426,   1458,   1472 
Generation  of  electricity  in  a  dynamo, 

i 105-1118 

Generators    1100-1269 

Geometry     for     studying     alternating 

currents     1485-1499 

German    silver     340,  345.   355 

German  silver  wire 355,  448,  460 

Gilbert     247,  248,   250,  412,   727 

Glass,  conductivity   33 

pierced  by  a  spark 132 

specific  inductive  capacity    122 

Glower,    Nernst    lamp 469 

Glue  pots,   electric 444 

Goldleaf    electroscope     23-25 

Gordon   cell    628 

Gore  pyro-magnetic  generator ;     412 

Gram-calorie     221 

centimeter    1309 

Gramme    205,   1306-1309 

ring  armature    1114,   1117.    1304 

Graphite    33,   505,   511 

Grasshopper  fuse    442 

Gravity     208-2 1  o 

battery    626 

Greek  derivation  of  electrical  terms, 

2,  202,  205,  208,  209 

knowledge  of  electricity  and  mag- 
netism     2,  9 

letters  used   245-247,   1450 

Grenet  cell   623 

Grid   battery    636 

Ground    803,  959,   1128,   1338 

detectors    801-803 

Grounds  for  static  electricity 63,  76-84 

155,   156 

from   static   electricity 63,  82-84,  94 

Grove  cell    634,  635 

Gulcher   thermopile    404 

"  H  "    725,  777 

Hair   and   static   electricity.  .  19,  61,  88,   no 

springs     838,  840 

Half-arcs    488,   1 143 

Hammers,  magnetic   782 

Harmonic    current    300 

Hartman-Braun   instruments    807 

Hautef  euille    506 

Hayden  cell 616 

Health    and    electricity.  ..  .51,  67,  70-73,  90 

Heat  and  work 220 

Heaters     444 

Heat  from  a  current 414-513 

Heating  effect  avoided    416 

importance    414 


Heating   used   for   measurement. ..  .804-814 

of   a   dynamo 1177-1179,   1240,   1241 

pads     444,  456 

Heat  lightning    150 

source  of  electricity 400-413 

units    221 

Helix   708 

Hemorrhage   465 

Henry,  Joseph    12,  233 

Henry    233 

Hertz    12 

High   bar  in  commutator 1213 

frequency     1433,   1434 

resistance   for  voltmeters 850 

voltage   dynamos    1220 

Hoang  Ti   9 

Hoho    heating    effect 496,  499,  500 

Holes  pierced  by  electricity. 132 

Hopkins     654 

Horizontal  component 244 

Horse-power    212,  214,  216,  217,  219 

hour    210,  219 

Horstman  &  Tousley 376 

Howell   lamp   indicator 810,  811,  814 

How  E.   M.   F.  is  generated  in  dyna- 
mos and  alternators    1114 

(See  also  E.  M.  F.) 
How  static  electricity  is  produced, 

it   16,   19,  35-42 

Humming  of  wires 153 

Hunning's  transmitter 347 

Hydro-electric   heating    500 

Hydrogen    612,  615,  616,  619,  628,  629 

632,  634,  637,  649   (7) 
Hysteresis      760-766   1241 

lala  wire    345 

Idle  current    1481-1484 

Ignition  by  spark 43'57,  64-67,    133-134 

Illinois  Steel  Company  magnets 789 

Immersion   coil 456 

Impedance    1501 

Impressed  E.  M.   F 1502 

Incandescent  dynamo    (see  Shunt  dy- 
namo), 
lamp    .324,  444,  468-482,  932,  959,   1157 

lamps  and  static   electricity 95,   125 

128,  129 


Inch 


204 


Inclination  compass   776 

Inclined  coil   instruments 855 

Inclosed    fuse 435 

Increase   of   temperature   of   a    gener- 
ator         462 

Indicator  for  static  electricity   20-24 

current 845,   1216 

polarity     310-313 

potential    810,  814,   1229 

Induced  E.   M.   F.    (see  E.   M.   F.). 
Induction  coils   for  gas  lighting,   49-57,   133 

electromagnetic    49-53 

furnace    503 

motor    1483,   1499 


410 


INDEX. 


Inductions,  unit  of    233 

Induction,    electrostatic     40,  99-103 

studied  with  galvanometer 1 1 10 

Inductor    1460 

Influence  machines   38,  40 

Ink    writing    register 1008 

Instantaneous    values    of    current    or 

E.    M.    F 1504-1509 

Instruments    800-856,  900-964 

abuse    of    903 

affected  by  stray  field 917-919 

calibration     849 

effect  of  stray  field 918,  926 

weakened    field    913-915 

electro-chemical    815-821 

electromagnetic     822-856 

electrostatic     801-803 

hot   wire    804-814 

inclined  coil   855 

scales    843,  847,  852,  853 

use   of    900-964 

(See  also  Ammeter,  Galvanometer, 
Measuring  instruments, 
Ohmmeter,  Tachometer, 
Thermometer,  V  o  1  t  m  e  t  er 
Wattmeter.) 

Insulation    1117,   1239 

magnetic     746-757.   1 1 19 

resistance    115,  239,  938 

testing    954-955 

Insulators   and   conductors 32-35 

Insurance    rules     45,372,416-418 

Internal  resistance  of  cells 633 

International  Congress  of  Electricians, 

225,  234 

electrical  units   225-233 

Inter-pole      1245 

Inverted    rotary    1512 

Iodide   of    potassium 312 

Ion      631,  632,  644-650 

Iron  beams  and  plates,   cutting 502 

Iron    for   magnets.  .. 715-743,  759-767,   1004 
1115,   1117-1119,   1174,   1320 

Iron-clad   instruments    753 

magnets  and  motors 743 

Iron   conductors    448 

fuses    426 

ore  concentrator 792 

locating     781 

Jablochkoff   generator    413 

Jacques  cell    4*3 

Jerusalem    Temple    10 

Jets,  gas   43,  54-57 

Jewel    bearings    912 

Joule     210,  213,   231,  415 

Jump    spark    gas    burner 43,  48 

Keeper,    magnet    788,   1004,  1006 

Kelvin   (see  Thomson,   Sir  William). 

electrostatic    voltmeter    802 

Kendall    generator    413 

Kettles,    electric    444 


Key,    telegraph    1006 

Kick  of  magnet  coil 1239,  1338-1342 

Kilogram    206 

meter    210 

Kilometer     203 

Kilowatt    213 

hour    218,  219 

Kinds  of  batteries 610 

current    300 

electricity    4 

magnets 743 

Knee  of  magnetization  curve 730 

Knox   376 

Kohlrausch  electrodynamometer    830 

Labels   and   static  electricity 96 

Laclede  battery 615 

Lag    1478 

Lagrange    496,  500 

Lamination    778,   1118,   1404,   1460 

Lamps  (see  Arc,  Incandescent). 
Latin  derivations  of  electrical  terms    ..202 

1224 

Laundry,    electric    456,  458 

Law  battery   615 

Law  of  fusing  currents 430 

magnetic    attraction     831,  832 

Laws  of  electrolysis 646-649 

Lead   storage   battery 635-642 

peroxide     635,  637 

sulphate     637 

Lebeau     506 

Leclanche   cell    615,  616 

Legal    units    234-236 

Length,  units    202-204 

Leveling  of  instruments 916 

Leyden   jar    in,   112,   124-128,  605 

Lifting  magnets    714-743,  789 

Light   (see  Arc,   Incandescent). 

Light  .from   static   electricity 127-131 

greenish     493 

violet      489 

Lighting     468-495 

gas  by  electricity 42-57,    134 

railway    trains 638,  641,   1160 

stations   and   static   electricity 93-95 

'  streets    337,  470,  488-493 

Lightning     107,   131,   132,   148-151 

I54-I57>   1206 

arresters     (see     Lightning     protec- 
tion). 

protection    10,   104,   107,   132 

155-157,  44i,  442 

Lime    507 

Line  of   force    (see  Magnetic  line   of 
force). 

Local  circuit   640,   1007 

currents    618,  619,  625,  626 

Locating  grounds   964 

magnetic   masses    780 

open  circuit    1 129 

Lodestone     772 

Long-shunt  dynamo   1247 


INDEX. 


411 


Loose   connections    1 1 95 

Loosening  fuses   429 

Low  bar  in  commutator 1214 

Luminous   effects    127-131,   151 

Lyndon    654 

McBerty   fuse    441 

McGee   generator    412 

McMillan    654 

Machine      (see     Dynamo,     Generator, 

Motor)    1427 

Magnet     7*7,  718 

(See  also  Electromagnet,  Iron  for 
magnets,  Permanent  mag- 
nets.) 

kinds   of    742-744 

steel    (see   Permanent  magnets). 

Magnetite  arc  lamp 492 

Magnetic   attraction    715-722,  831,  832 

brakes    793 

circuit     726-733,  742-767,   1119 

clutch    785,  788 

field    244,  704-7 1 * 

varying     1 1 63- 1170 

Magnetic    force    241,  712-722,   1305 

flux    241,  249 

hammers    782 

indicator  for  current  direction....      310 

induction    49'53 

instruments    822 

intensity    242,  730 

leakage     745-758,   1119,   "65,   1206 

line    of    force 241-243,  702-730,   1163 

permeability    724.   725.   735 

phantoms 7°4-7°9 

pole    243,  246 

pull    836,  914,  917-920,   1006 

reluctance    1167-1169 

ring    783 

saturation     722-726 

screwdriver 782,   1 1 1  o 

separator    791,  792 

shield     75i 

storms 151,   152 

survey     .' 781 

traction    719,  720 

vane  instruments    856 

Magnetism   and   electricity    (see   Elec- 
tromagnet). 

first   known    9 

terrestrial    ..151,   152,   241,   242,   774-777 

Magnetizing   circuits    of   dynamos 447 

733-739,   1124,   1137-1140,   1149-1153 
1170-1173 

Magneto    955,   1015,   1125-1131,   1160 

Magnetometer    781 

Magnetomotive  force 246,  247,  250 

733-735 

Main    line   cut-out 434 

Manganese    dioxide    616 

Manganin    345 

Manhattan  lamp  regulator 1426 

Marble    33,  440 


Mason    40 

Matches   and    electricity 66 

Maxwell     12,  241 

Mean   values    1501 

Measuring  current  (see  Current  meas- 
urement). 

efficiency    1316-1318 

E.  M.   F.    (see  E.  M.   F.  measure- 
ment). 

instruments     752-755,  800-856 

and   magnetic   fields 752-755 

electric  motors    824 

radiant   energy    407 

resistance    935-959 

speed     1187 

static    electricity 135-147 

temperature     408,   1178 

torque     1308-1313 

work     1314 

Mechanical    depolarization    629 

Medical    belts    70,  71,  73 

coil     51 

Megohm    239 

Melting   fuses    427 

pots,  electric   444 

Menges,   pyromagnetic  devices    412 

Mercury    619,  620,  625 

resistance    226,  238 

vapor  lamp    468,  486,  493 

Mesh-connected,  three-phase  armature, 

M50 

Metallic   flaming   arc 492 

Meter    202 

(See    also    Ammeter,     Instrument, 
Ohmmeter,  Voltmeter,  Watt- 
meter.) 
Metric  system  of  units 200-221 

Mho    334,  335 

Mica     112,   114,   122,   123,  441,  455 

Microfarad   139 

Microhm     240 

Mile   204 

-ohm    357 

Mil-foot     344 

Miller's    magnets    791 

Milliampere    329 

Millimeter    203 

Millivoltmeter     922,  926 

Mist  and  static  electricity 97 

Moissan    506,  5 1 5 

Molecules      644 

Monocyclic  A.   C.  system 1*53 

Montauk   cable    443 

Moore  light 130 

Moses    10 

Motor    447,   1000-1013,   1300-1388 

arc  light  circuit 1323,   1326,   1329 

battery    1300-1303,   1323,   1325 

cause  of   rotation   .....1300,   1302,   1303 

1305 

cumulatively  wound 1383 

differentially   wound    1382 

elementary     1000-1014 


412 


INDEX. 


Motor  E.  M.   F 1347-1350 

generator    1264 

oblique  approach   1300-1302 

railway    1323,   1324,   1327,   1385-1387 

regulation    1381-1387 

series      1302,   1303,   1321-1328 

1382-1387 

shunt     1321,   1329-1346 

toy     1300-1303 

Multicellular  voltmeter    144 

Multiphase    1442 

Multiple  arc    308,  331 

circuit     308,  331,  332,   1157 

operation   of   dynamos 1222-1236 

point  switch    44 

series    336,  337 

Multipliers    931 

Multipolar    machine.  .  1121,   1182,  1431-1433 
Multi-tooth    winding    1431 

National  Electrical  Code 416-418 

Nature  and  electricity 10,  97,  148-157 

Needle  141,  143,  704,  790,  1505 

Negative  608,  644,  649 

Nernst  generator  413 

lamp  469 

Neutral  plane  13 77 

New  British  wire  gage 358 

New  York,  earth's  magnetism 244 

Niagara  Falls  1119,  1519 

Nitric  acid  154,  631 

Nitrogen  fixed  by  electricity  511 

Noe  thermopile  404 

Nominal  candle-power  1 144 

Non-magnetic  watch  750 

Northern  lights  151 

North  pole  1122,  1123 

"  No  voltage  "  release i334>  1335 

Number  of  paths  in  armature 1182 

poles    1 1 22 

Obtunding    465 

Oersted     250 

Ohm    225,  226 

B.  A 237 

George    Simon    226 

international    226 

legal     234 

Ohmic    drop    361,  366,   1497 

Ohmmeter    946,  947 

Ohm's  law   314-326,  375,  415,  633 

649   (14),  936,  950,   1154,   1349,   1508 

for  alternating  currents 1501 

Open    circuit    610,   1127,   1129,   1195 

battery    611-625 

coil   arc   dynamo 1161,  1431 

Oscillatory  discharge   132 

Oscillograph    913,   1505 

Output  of  a  dynamo     •. 1 1 54 

Ovens,    electric    444,  465,  499,  503 

Over-compounded   dynamo    .....1151,   1152 

1244,   1245 
Overload   release    1337 


Oxygen    507,  624,  634,  649   (15) 

Ozone     66-69,  86,  649   (15) 

Pads,  electric  783 

Panel  board 440 

Paper  and  electricity 19,  74-78,  92 

Paraffin  33,  53,  112,  114,  119,  122 

Parallel  circuits  331 

Paris  200,  234 

International  Congress,  1900... 241,  242 

Partz  bichromate  cell 624 

Pasted  cell  635 

Peltier  effect 409 

Pendant  gas  burner 54,  56 

Pendulum  207 

Pennsylvania  R.  R.  train  lighting 1160 

Permanence  778 

Permanent  magnets. 759,  769,  770,  833-840 
912-918,  1124-1130,  1320-1322 

Permanganate  of  potash 623 

Permeability  724,  725,  735,  740,  741 

1167,  1174,  1320 

Permeameter  740,  741 

Peroxide  of  lead 635-638 

Petterson  506 

Phase  difference  1442,  1493,  1502 

Phosphorus  505 

Photography  ,  1504 

Photometer  48o 

Physicians'  use  of  electricity.  .301-306,  467 

"Pi"  245-248,  719,  733,  1307 

Piercing  holes  by  electricity.  .  .94,  95,  132 

Pilot  lamp  1 340 

Pine  trees  and  electricity no 

Plante  cell  635 

Plants  33,  no,  141,  154 

Plasters,  electric  783 

Platinized  silver  629 

Plugs,  fuse  424 

Plunge  battery  624 

Points  and  static  electricity 101-107 

Polarity  changer  1114 

indicator     310-313,  639 

of  a  battery 639 

Polarization  ..611-613,  649  (10),  649  (14) 

651-653 

Polarized  apparatus  787,  1015 

Pole-changer  1114 

Poly-odontal  winding  1431 

Polyphase  1442 

Poor  contact,  trouble  from 1 195 

Poole  376 

Porcelain  33,  435 

Porous  cup  631 

Portable  gas  lighter 47 

Porter  motor 1303 

Positive  and  negative  charges.  .  .25,  26,  29 

644 

terminals  609 

Potassium  idodide  indicator 312 

Potato  test  313,  639 

Potential  363 

indicator     810,  814,   1229 


INDEX. 


413 


Pound-feet     1309-1313 

Pound     mile-ohm    357 

Pounds   pull    1306-1309 

Power     212-217 

factor    "35,   1483 

of  a  motor 1351-1388 

Precautions  in  handling  a  dynamo   ...1220 

1239 

measuring    944.  945.  951.  963 

use  of  instruments 9°3-920 

Preece,  law  of  fusing  currents 430 

Pressure,  electrical    (see  E.   M.    F.). 

analogy    3°8 

Primary   batteries    604-634 

Prime    conductor    39 

Principle  of  dynamo 1114 

electromagnetic  instruments    837 

transformer    i4°5 

Printing  offices  and  electricity.  ..  .64,  74-80 

Prony  brake   1313-1316 

Protected  fuses 429,  435,  436 

Protector,    fuse    424,  442 

Pull  of  a  magnet 714-722 

on  an  armature 1305-1313 

a  wire   1306 

Pulsating  current    ...307,   1015,   1016,   1114 

Push    button    54-57.   1010 

Pyro-electric  effects   410 

-magnetic    effects    41 

generators     412 

Quadrant   electrometer    145 

Quadrature    1461,   1477 

Quantity  of  electricity 135-137,   140 

Quarter-phase    1437,   1442 

Queen  instruments    807,  856 

Radio-activity     654 

Rails,    electrically    welded 498 

measuring  current  in 923 

Railway    motors    1309 

train   lighting    638,  641,   1160 

Rain  and  static  electricity 97 

Range  of  instruments.  ..  .802,  931-933,  941 
947,  952-955 

Ratchet  gas  burner 54,   56 

Rating  of  fuses 427-432 

Rats    and    electricity 126 

Recording    telegraph    receivers 1008 

voltmeter,  Bristol    830 

Refining   copper    649   (9) 

Refrigeration,  electrical    409 

Register,  telegraph   1008 

Regulation   of    dynamos.  ...  1 149,   1158-1170 
1214,   1232,   1233,   1240-1242,   1463,   1466 

of  arc  lamps   488 

Regulator,    A.    C 1426,   1463 

dynamo     1214,   1223,   1225 

Relation  between  chemistry   and  elec- 
tricity      600-653 

current    and   magnetism.  ..  .726,  700-793 
1107,   1108 

electricity   and    heat 400-514 


Relation  between  power  and  C.  E.  M. 

F 1357,   1362 

Relay    1005,    1007 

Reliability  of   instruments 910 

Reluctance     249,  250,   1167,   1168 

Remanence     761 

Removal  of  magnetism    749 

static   electricity    76-84,   92 

Repulsion,   magnetic    712,   787 

by   static    electricity.  17,   22-25,   87-92,  97 

98 

Requirements    of    fuses 425,  427- 

Residual    charge    123 

magnetism     766,  767,   1172,   1199 

1205-1212 
Resistance     308,  348 

affected    by    pressure 346 

affected  by  temperature 340,  345 

1178,   1475 

measurement     935-959 

of   batteries    633 

magnet   coil    739 

.voltmeter     939 

standard     956-958 

unit  (see  Ohm). 

Resolution    of    forces 1485-1492 

Return  circuit,  gaspipe 45 

Reversed    dynamo 1201,   1208-1212 

1236-1238 

Reversible  thermal  effect 409 

Reversing  connections  ...   1196,   1199,   1201 

1203 

motors     1 388 

switch    1016 

Revolution    counter         1186,   1187 

Revolving  field  alternator 1458-1463 

Rheostat      444,  446-454,   1149,   1171 

1225,   1235,   1243,   1466 

Ring   armature    1117 

Rise  of  temperature 419 

Ritter   battery    635 

Roebling  wire  gage 358 

Rotary  converter    1429,   1510-1520 

Rotor     1427,   1428,   1460 

Rowland,    H.    A 220 

Rubber 33,39 

Rubbing,    electricity    by.  19,  27-29,  38,  39,  47 

(See  also  Static  electricity.) 
Rule  of  magnetic  attraction 831 

thumb     831,   1 1  it 

underwriters      416-418,  425,  428 

Rutherford   654 

Sad  iron,  electric 456 

Safety,    catch     424 

coil     1425 

fuse     424 

Sal  ammoniac    435 

cells     614-622 

Salient    magnet    pole 744 

Salt    33,  508,  649   (7) 

Samson  cell    616 

Sand    508 


414 


INDEX. 


Saturation  of  iron 722-726,   1 1 74 

Sawdust    508 

Scales     847,   1312 

Schwedoff   generator    412 

Screwdriver,    magnetic    782 

Second    207 

Selective    ringing    1015 

Self-excited   machines    1 139 

inductance     1495-1497 

Separately   excited   dynamo 1137 

Series    circuits 308,  327,  330,  470,  490 

959,   1426,   1460,   1472 

dynamo    1139-1145,   1217-1221 

field    coil     1139,   1140,   1150,   1321 

1381-1383 

motor     1321,   1323-1328,   1385-1387 

multiple     336 

Setting  dynamo  brushes.  ..  1 162,   1189,   1191 

Shellac     33,   53.   1216 

Shifting  brushes    1162,   1191 

Shock      19,   37,   51,   74,    124-128,  363 

364,   1220 

Short   circuit    439,   H95,   "96,   1205 

1206,   1338 

shunt    dynamo    1 246 

Shotgun   diagram    479 

Shunt     ..331,  488,  649    (n),  839,  840,  843 
963,   1146,   1183-1185 

dynamo    736,  737,   1146-1149 

1171-1177 
field     coil.. 1139,   1146,   1149,   1321,   1381 

motor    736,  737,   1321,   1329-1346 

1384,   1386 

Shunted   ammeters    839-842 

Siemens  armatures   1117 

electrodynamometer     .823,  825,  830,  916 

unit    of    resistance 238 

Silica 508 

Siloxicon     505 

Silver    33,  227,   317,  621,  629 

chloride   cell    621 

voltameter    227 

Sine  curve    304,   1498,   1499,   *S13 

Single-phase    1431.   *44i 

Single-pole  cut-out    439 

-stroke  electric  bell    101 1 

Sinsteden  battery 635 

Sinusoidal   current    304,   1401 

Siphon  recorder   . 91 

Six-phase     1442 

Size  of  armature  wire 1180 

conductors      349-359,  369'374>   738 

S.   K.   C.  alternator 1460 

Slate   slabs   in   rheostats 449 

Slip    rings    1427 

Slotted   armature    1117,   1304,   1431 

Smashing   point,   incandescent   lamp..     477 

Smee  cell    629 

Smell   from  electricity 17,  66 

Smooth    core    armature 1117,   1304 

Smoothing    iron,    electric 444 

Soda     500 

Sodium    505 


Sodium    bichromate    624,  625 

Solder     430,  441 

Soldering   copper,    electric.  ..  .444,  455,  461 

Solenoid    708 

Source  of  voltage  of  dynamo 41 

electric  current   1 100 

error    932 

Spark    at    brushes. .. i 193,    1213,   1216,   1377 

at   switch    1 239 

coil     52,   53,   1135 

electric    17,    19,  42,   50,   51,   131-134 

Specific  inductive  capacity 118-122 

resistance    340 

Speed  of  dynamo   1175,   1186,   1187 

motor    1370,   1373-1375,   1380-1387 

Sperry  arc  dynamo 1 1 62 

Spider,    armature    1117 

Spinning  and  electricity 92 

Sprinklers,  fire    443 

Square    mil     342 

Stamp  cancelers,   electric 444 

Standard    resistance    956-958 

Standard  wire  gage   358 

electrical    225-227,  925,  926 

Stanley   alternator    1460 

Stanley-Chesney-Kelly     1460 

ground   detector    803 

Star  connections    1451,   1452 

Starting    motors.  1325-1327,   1334-1345,   1368 

series  dynamo    1221 

shunt    dynamo     1172,   1173,   1223 

1225,   1229 
Static    (see   Electrostatic,   Capacity). 

charge  of  ions 644 

electricity    4-140,  302 

importance  of   13-15,   35,  42-58 

in  conductors   31-36 

effects  of    17,   19,  74 

how  detected   17-24,  61 

measured     135-147 

produced     ...16,   19,  27-29,  38-42 
47,  59 

removed     76-84,   104-110,   155 

156 

on    telegraph    lines. 91,   107,   116,   157 

telephone    lines     ....100-103,   107 

115,   116,   157 

relation   to   current 4-6,   158 

uses  of    35,  42-58 

transformer    1520 

Stator    1428 

Steam   and   static   electricity 76,  77,  92 

Steel     for    magnets     (see     Permanent 

magnets). 
Step-by-step    measurement.  1405,   1505,   1506 

Step-up   transformer    345,   1416,   1459 

Sterilizer     465 

Stopping  motors   1328,    1333-1345 

Storage    battery 604,  605,   635-642,   654 

1254,    1261 

Stoves,    electric 444,  456 

Stray    magnetic    field     (see    Magnetic 
leakage). 


INDEX. 


415 


Street    lighting 337,  470,  485-492,   1425 

String   of   lamps 959 

Submarine  cables    91 

Substances  used   for  fuses 426 

1  Sulphated   cell    637,  640 

Sulphions      623,   632,  649    (5) 

Sulphuric     acid. 623-625,  629,  631,  636,  637 

Surface   electricity    36 

Surgeons  and  electricity 444.  466 

Surging     I431 

Sweeping,  electrolytic    650 

Switch,    multiple   point 44 

Switchboard    752,   1435 

Synchronous    converter 1511-1519 

motor     1483 

Systems  of  units 200,  224 

Table,    ampere  turns 735 

carrying   capacity   of   wires 418 

electrical   cooking    457 

electrical    furnace   products 506 

electrochemical  equivalents    648 

fusing    currents    430 

incandescent    lamp    performance..      475 

476 

instrument  calibration   929 

magnetic    traction    719 

specific  resistance   340 

wire   gages    358 

resistance    347 

Tachometer    1 186 

Tailor  shop  and  electricity 64,  65 

Tantalum     469,  673,  481,  482 

Taps    1416,   1463 

Target  diagram 479 

Tea  kettle,   electric 444,  456 

Teaser    coil     1153 

Telegraph    405,  626,   1005-1007 

and   static   electricity.  .91,   107,   116,   151 

152,   157 

humming  of  wires   153 

line    328,   1006,   1007 

relay    1005-1007 

sounder     .711,   1005,   1006 

Telephone     347,  441,  442 

and   static   electricity. .  .99-103,   107,   115 
116,   151,   152,   157 

call    bells    1125 

exchange     641 

receiver IO05 

wire,    resistance    356 

Telephone   cables    1 133 

Temperature   and    resistance. .  .345,  810-814 
coefficient    345,  810-814 


of  arc 


494 


dynamo,   determined 1178,   1216 

incandescent  lamp   481 

Tempering  electrically    444,  499 

Temple  at  Jerusalem Io 

Terrestrial    magnetism 151,    152,   241 

244,   774-777 

Tesla    130,  412 

Testing  by  magneto.  .  .955,   1126-1131,   1133 


Testing   by   lamps 959 

with    voltmeter    H99 

Thales     2,   i  o 

Thawing  frozen  pipes 465 

Theophrastus    411 

Thermal    arrester    441 

Thermo  couple 402 

electric    currents    402-409 

Thermometer    222,  223,   1178 

Thermopile     402-410,   1 100 

Thermostat     816 

Thomson,   Elihu    412,  496-499,  855,  913 

-Houston     412,  855,  913 

Sir    William    ....144,   145,  801-803,  830 

Three-phase    alternator 1449-1451,   1460 

1461 

circuit     1449-1453 

current    1440-1442,   1516 

Three-wire    system.  .  .439,   1122,   1443,   1452 

Throwing  a  dynamo  in  service    1221, 

1225-1231 

out   of  service 1234,  1233 

Thunder    40,  97,   131,   148-157 

(See  also   Lightning.) 

Time  and  power 215 

Tin  foil    111-113 

pail   experiments    37,  99 

Toepler-Holtz  machine 38,  40 

Toothed  armature    1117,   1 304 

Torque    788,   1309-1312,   1378,   1379 

Torsion  head   829 

Torus    708 

Tourmaline     411 

Traction  curve  and  table 720 

Train  lighting 638,  641,   1160 

Transformer     (see     Alternating     cur- 
rent transformer). 

Transient  current    140,   141 

Transmission    dynamometer     1312 

Transmission   lines    U35>   1452 

Transposition  of  wires 103 

Trolley    car    363 

(See  also  Motor,  series.) 

Troost    506 

Troubles   with    dynamos.  ...  1 185,   1194-1197 
1210-1216,   1255,   1259 

Tungsten  lamp    469,  473,  481 

Tunnel-wound  armature   1304 

Two-path    winding    1 182 

Turbo-generator    143  5 

phase    alternator    1439,   1448 

currents     1442 

Undercompounded   dynamo    1255 

United  States  Navy  rule 417 

Units     129-132,  200-251 

electrical     224-241 

electromagnetic     136,  224 

electrostatic     136,  224 

English     ....201,  204,  206,   210-212,  214 
217,  222 

heat     218-221 

force    208 


416 


INDEX. 


Units,   international  electrical    225-233 

length     .  . . . 202-204 

magnetic    242-250 

metric     200-22 1 

power     213,  214,  216,  217 

time    207 

weight     205,  206 

work   209,  2 1  o 

Uni-tooth  winding    1431 

Use  of  alternating  current 1402 

ammeter  as  voltmeter 903 

arc     488-495,  501,  502 

batteries    604 

dynamos    1103 

electromagnets      784-793 

heating  effects    444,  417-513 

magnetic    attraction    785-793 

repulsion     787 

permanent   magnets    779-783 

series  dynamos    1141 

shunt  dynamos    1 148 

static  electricity   14,  35,  42-58,  86 

91,   107,   no,   116,   132,   134,   154 

storage  batteries   642 

voltmeter  as  ammeter 904 


"V"      777 

Vacuum   in   lamp 469 

Valency     648 

Variable    resistance    347,  450,   1170 

Varying  magnetizing  current 1170 

Vegetation  and  electricity no,   141,   154 

Velocity    208 

of    light    224 

Ventilation  of  armature H79 

Vertical    component    of    earth's    mag- 
netism     244,  777 

Vibrating  bell    1010 

Vibrator    1008,   1015,   1016 

Volt,    international    138,  225,  228 

legal     235 

selector    930 

Volta     ii,   138,  228,  603 

Voltage  (see  E.  M.  F.). 

measuring  small    933 

of  a  shunt  dynamo 1176 

series  dynamos    1 145 

Voltaic  battery    603 

Voltameter    815-821 

Volt-amperes    1483 

Voltmeter,    double-scale    847 

electrodynamometer    826 

electromagnetic     835-837,  846-856 

902-908,  928-945 

electrostatic     143-147,  801,  802 

for  measuring  resistance.  .  .936-944,  956 

hot  wire 806,  807 

Howell     814 

how   different    from   ammeter 848 

voltameter     846 

multiplier    93 1 

switch 842 


Voltmeter,    to    measure    instantaneous 

E.  M.  F 1507,  1508 

transformer    1417 

with  low  resistance    851 


Warren   alternator    1460 

Washburn  &  Moen  wire  gage 358 

Watches  and  magnetism 748-75 1 

Water,   analogy  to   electricity 308 

as  a  standard  weight 205 

conductivity  of   33 

density     205 

effect    on    carbide 507 

heating     221,  444,  456,  465 

pail   forge    499,  500 

pipes  thawed  by  electricity 465 

rheostat     448,  451-454 

Watt  213,  216 

international     232 

(See  also  Measuring  Power,  Work.) 

James    213 

-hours    218-221,  818 

wattless  component     ...1481-1484,   1492 

current     1481-1484 

Wattmeter    827,   1425 

Weather  variations  in   electricity.  ..  .60,  80 

Weaving  and  electricity 92 

Weber     830 

Weight    212,  215 

units   205,  206 

Welding,    electric    444,  495-502,   1403 

Welsbach   mantle    469 

Western    Electric    arc 1 162 

Western   Union  wire  standard 357 

Westinghouse  booster    1472 

instruments   802,  854 

street    incandescents 337 

Weston    instruments     ...752,  823,  830,  835 
840,  914,  941,  963 

What    is   electricity i 

Wheatstone    bridge     403,  449,  948-954 

Whirligig,    electric    104-106 

Whitney  instruments    806,  914 

Wiechman     654 

Wimshurst    machine     38,  40 

Winding  armatures, 

A.    C 1431-1433,   1438-1441,   1459 

D.    C 1117,   1300-1304 

Wind    from    electricity 104-110 

Windmill  electric  meter 808 

Wiper    1 507 

Wire  gages   358,  359 

gauze  brush    1 192 

size    determined 349-359>  369-374 

738 

table    .'. 348,  372 

humming    of    153 

Wirt  brushes 1 192 

Wohler    506 

Wood  arc  dynamo 1 1 62 

Wooden  electrodes   35 

Work   done   by   motor 1314-1319 


INDEX. 


Work    in    electrolytic    cell 650 

unit    209-2 1 2 

Wright     515 

Wright  demand  meter 808,  809 

Wrong  connection  of  instrument....      905 

connections    1196,   1199,   1201-1203 

use    of    instruments 903-912 

Wrought  iron  (see  Iron  for  magnets). 


"Y"   connection    1451,  1452 

Zerner   blowpipe    501,  502 

Zinc     ....608,  609,  614,  615,  618,  619,  626 
629,  632,  649   (n) 

chloride    615 

irregularly  eaten    618-620 

sulphate    624 


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